How often should I water these seedlings in coco peat perlite? (90 degree temp, RH 50%)

Always keep wet. Coco is a inherit substrate. So a coco grow is Hydro in a bag. Ph 5.8. 83 F is max temp. Your leaves will start tacoing as it gets bigger if you dont bring the temp down. You also don't want anything slow release. Coco is heavy irrigation so your plants get the boost of a Hydro grow. Cal mag is also needed with Coco. The one negative of Coco is it requires extra cal mag to grow in. If you try to grow like it is in soil, you can. Just there will be issues, and frustration later. Liquid nutrition is required for fast absorption, and fast growth for Hydro. Overwatering is not possible in coco perlite mix unless you leave it under a steady stream. Ph 5.8 in ph 5.8 out in run off. This allows full access to all of the coco mix for roots to work with, and no lock out zones.