It depends what you are doing. Unfortunately there have been cases where students have had a bad day's flying, and have left without paying their bills never to be seen again.
Because most FTOs have been burnt in the past they often ask you to keep an amount in credit at the start of each day that roughly covers what you would fly off in a day. If you are doing IR training you can easily get through a grand in a day so to expect you to have that in your account in the morning is not unreasonable. If you are hour building at £50 an hour it is unreasonable.
Whatever you do, always pay by credit card, that gives you the most effective protection.