How often should I water plants in party cups?

Put your finger about an inch into the soil and see how they feel. If they're dry, water them. If you can feel moisture, check on them tomorrow. Also for seedlings, you should try misting or at least try to be gentle watering them as to avoid damaging roots.

 

Alternatively, it's very easy to do the lift method with party cups. Water your plants until you get run off. Pick them up. Water them again when they feel really light.

You'll want to keep your seedlings pretty moist (not mud and no standing water) but as soon as your true leaves start coming in, let it dry out between waterings so your roots grow towards the water at the bottom of the cup.

Make sure you have proper drainage and you can't over water in one sitting. You'll see problems if you water too often though.