I tried asking Skinny about this but gave up waiting on the phone (a ploy I bet).
If you purchase $4 on a Monday say for 60 minutes and have used it all up by Thursday, can you purchase another $4 for the rest of the week?
Silly example but really I was thinking of the data addons and if you ran out during the week would you be stuck with ad hoc data rates until the next time your top up cycle arrived?