It depends which Bank you're with, as charges vary daily, weekly, and monthly....personally, I would either phone your bank, or better still, if you have time, make an appointment to see an Advisor in your local branch to discuss what exactly the charges will be, if you do stray into being overdrawn....of course, you could ask them to give you an overdraft on your account, for this to be less likely. Good luck, fruitsalad.
The link below helps you to understand how overdrafts work and includes how to keep an eye on your outgoings, so as not to incur charges on your account, if you already have an overdraft. Hope it helps.