I can't say how often as it depends on a variety of factors. The ambient temperature and humidity will play a role in how long the coco stays wet. If the plants were bigger and actually taking up any significant water that would be a factor as well. Those seedlings are not drinking much of anything right now. That coco looks really wet like you could squeeze a handful in your hand and a bunch of water would come out. Could take a couple days for the coco to dry out some. You won't get salt buildup by letting it dry out some. But you do not want to let it dry out completely ever.
With a plant that size in pots that size you can wait until the top just starts to dry out and then water around the plant. You don't have to soak the entire pot at this time. The coco will still be wet down inside. When the plants get bigger and you see roots at the holes in the bottom of the pots then it's game time.
I grow in coco using fabric pots. I don't use perlite but grow in 100% coco. Here's a shot of a couple plants showing what the coco looks like when it's good and damp but not soaking wet. If squeezed you're hand will be moist but no water will squeeze out except for a drop or two. But when the plants are bigger and have a well developed root system you can keep them wetter than these. This was right before watering
