Frequency:
* Frequently
* Regularly
* Usually
* Commonly
* Generally
* Repeatedly
* Periodically
* Habitually
* Customarily
* Invariably
* Consistently
* Always
* Many times
* Time and again
Informal:
* A lot
* Lots
* Quite a bit
* Pretty often
* A good deal
* Now and then
* From time to time
* Once in a while
The best choice will depend on the specific context and desired level of formality.