When you buy something with a debit card does the amount change online instantly or does it take a bit of time?

It depends how quickly the retailer submits the request for payment. The amount of the transaction is deducted immediately from your available funds, but the actual transaction will only show on your account once the retailer has asked for payment from the bank. For example... Last Friday, I used my debit card in Sainsbury's. The total value of the purchases was immediately subtracted from my available funds - but - the actual transaction didn't show until this morning.