Just to clarify, a rep is one. Like one sit-up. A set is the number of reps you do in the sitting. Like 100 reps per set.
And now...
What are your goals? Do you want a six pack or do you just want a good ab workout? These are the things you should state first.
If you are doing a lot of sit-ups and feel no stress, try different ab workouts. Try leg lifts, V-ups, crunches, weights, etc. The main thing about a workout is if you keep doing them, your muscles will get used to them and you will thus lose the work in the work out. Unless your goal is to hold the record for most sit-ups in the world, you should not increase reps (especially when you get to high reps) but change the exercise and come back to sit-ups later.
I think abs or the core is the most vital body part and should be worked everyday. Now this doesn't have to mean 500 sit-ups, but it does mean there isn't a day when you're not using it.