How often do I need to review the prospects?

As often as you like. Qualifier is designed so you never have to visit the review list: campaigns pick and contact prospects on their own.

If you want more control, reviewing is a good way to get it:

• Without required review, you can check the To be contacted list on the campaign's Prospects tab whenever you want and reject anyone you'd rather skip.
• With required review, you get an email when prospects are waiting. If you don't review within 3 business days, they're approved automatically.

The list is refilled regularly as prospects are contacted or rejected. It usually holds around a week of sending for the campaign. If it looks shorter than usual, check back in a few hours.