How thick should plywood be when building a desk?
Close to what you need, here's a shelf sag calculator. if you imagine each end is supported, a 4' shelf of 0.6" thick ply will hold 30 lbs without issue.

If you spent a couple more bucks and added a 2x2 frame around the underside of the plywood, it would be dramatically stronger. Here's a pretty simple example video (larger but you get the idea).