The answer, assuming you're not a bank or the government, is none at all. Let me explain.
A long time ago, banks used to issue their own bank notes. The banks promised to pay the bearer of the notes, on demand, the amount of "real" money that the note promised, say in silver coins. It was the bank that guaranteed the value of the notes, and if the bank became insolvent (i.e. owed more than what it was owed plus what it had in its vaults), the bank notes could not be paid in full. But since the advent of central banking, with Bank of England bank notes, that arrangement is long gone. Or is it?
What happens when you open a new bank account? Actually it's quite simple.
You go to your local branch of Lloydclay's Bank of Corporation, and ask to open a current account. They say, "certainly" (as long as you promise to deposit £1,000 per month). You decide to hand over two £10 Bank of England notes. What happens then? Are those notes stored somewhere for you, so that when you ask for them later, you can get them back?
No. That's not the way it works. What happens is that the bank makes a note in a book (or these days in a computer database) that it owes you £20. It then uses the £20 in Bank of England notes in whatever way it decides to. For example, it could lend them to someone else, or give them to someone else who wishes to withdraw £20 which they deposited earlier. What you've actually done is exchanged money (Bank of England notes) for bank credit. The bank promises to pay you money if you ask for it. But it can only do that if it hasn't lost the money which you deposited.
Imagine you're the first depositor. The bank then lends the £20 you deposited to someone who is setting up a company to manufacture pins. But instead of buying a pin-making machine, the borrower instead goes out, buys 10 pints of lager and drinks them. Now there's a problem. The borrower can't make pins, so he can't sell pins to make £21 to pay back to the bank (£20 plus interest). So when you walk into the bank to ask for your £20 back, the bank can't pay. So the bank credit is worthless. You exchanged some money (Bank of England notes) for a promise by the retail bank to pay you money later, but the retail bank can't pay you, so you lose. You'd have been better off stuffing the money in your mattress. Bank credit is directly analogous to a bank note issued by the retail bank - it is a promise by the bank to pay you in money (Bank of England notes) when you ask for it. But the value of the bank credit is entirely predicated on the ability of the retail bank to pay.
So having a positive balance in an account at a retail bank doesn't mean that you have money. All you have is a promise by the bank to pay you money if you ask for it and if the bank has enough money to pay you.
This situation is highly unsatisfactory - it (rightly) causes people to panic easily. If a rumour goes around that bank X only has £1 million, but owes £2 million to depositors, then people who rush to withdraw their deposits will get all their money back, while the slower ones will get nothing, so a bank run ensues. Two important safeguards are in place for (some) depositors:
1. Banks are required by law to maintain capital. This is a certain amount of money (or investments which can easily be sold for money, such as risk-free (cough!) government bonds) which the bank must keep in order to pay depositors in the event of losing money on their loans. In the case above, the bank made a bad loan to the supposed pin-maker, and so the owners of the bank (the shareholders) have to pay the £20 to you. As long as the losses do not exceed the capital, only the shareholders will lose money. But if the losses are greater than capital, someone else will lose money. This requires a second line of defence for the depositor:
2. In the UK, the Financial Services Compensation Scheme (FSCS) is a government-run guarantee of some deposits for individual and some other types of depositor. (Similar schemes operate in other countries e.g. FDIC in the USA). When a bank cannot meet its obligations, the government takes money from other banks and building societies to pay back the depositors of the failed bank, or if absolutely necessary takes money through taxation to pay.
I want to emphasise again: if the bank's losses are greater than the bank's capital, someone else will lose money. The only question at this point is who. Because the government has decided that depositors will be protected, the money is instead taken from other banks or building societies, or from taxpayers. This is actually a subsidy from good banks and building societies which manage their risks well and/or from taxpayers to bad banks which chase slightly higher returns with much greater risk.
In principle, the FSCS ought never to be invoked. The shareholders, who would lose their capital in the event of the FSCS being used, should prevent the bank from losing this much money. But if the bank's directors and employees can make high-yielding loans that look like a reasonable risk-reward compromise to shareholders in the short run, and they pay themselves big bonuses as a result, then the shareholders may not be able to organise themselves quickly enough to prevent the losses.
This blog has looked at money and bank credit. In a later post, I'll discuss the different forms of money. It's not just bank notes, you know, but it's not much more than that.
Update [2015-03-23]: Removed description of Bank of England notes as "real" money, as it is controversial, but not particularly important here.
My use of the terms "money" and "bank credit" have also lead to some controversy, as people have (rightly) pointed out that bank credit is a form of money. (In the UK, bank credit is counted in the M4 measure of the money "supply" or, as I prefer to say, the quantity or stock of money. What I have called money in my blog is the monetary base – money created by the central bank.) My intention is to use simpler non-technical terms here, since terms like "monetary base" used to make my eyes glaze over. But I do need to distinguish between central bank money and retail bank credit, since it is very important in understanding why the ability of banks to create money does not allow them to consume without producing.