Ideally, a review session should be done every day.
When starting out, finishing a review of your whole deck shouldn't be too difficult (depending on size, of course).
But you'll find that a complete review session will take longer and longer as you go on, since there'll be more cards to review each time.
Don't feel discouraged. Just try to keep the habit going. If you stop, your backlog of unreviewed cards will keep growing.