I have a shared hosting account with a 20GB monthly bandwidth limit. I have exceeded my monthly limit and according to my host my counter is never reset, they say they use a continuous 30 day counter.

So for example, I make payment on the 1st of each month, say I use 20GB in the last week of the month. My bandwidth counter is not reset on the 1st of the new month and my bandwidth will only become available in the last week of the new month.

Is this common practice by shared hosting companies?

Sounds a bit shady to me. Surely my counters should be reset on the 1st of every month when I make payment and 20GB of bandwidth should be available from the day payment is made?