how long does it take for purchases to be charged to a debit card

how long does it generally take for a debit card purchased to show up on my debit card billing? my other purchases seem to have been billed quite quickly, however i have 2 purchases 1 of which should not be able to be billed since after the first purchase my debit card will be low on funds. however both purchases was successful and neither has shown up in my debit card account charges, or has reduced my debit card funds.