It would take a couple of months or maybe longer on a specific program to get you to be able to safely overhead squat with a oly technique bar (5kg) depending what your mobility / flexibility is like now. Using a 20kg empty bar can cause injury if you are a beginner because of shoulder mobility / stability and total flexibility. You need specialist equipment for any loaded work (technique bars, technique plates, a platform, rubber plates).

I would say stick with a dowel pole like httk says and get to the point where you can do an overhead squat with just that and use it for maintaining flexibility/ warm up for olympic squats. Squats are quite technical and I think most people lack the flexibility and correct technique to even squat. I wouldn't call an olympic squat a basic exercise at all really and an overhead squat requires even more technique. I would get an over head squat assessment (see a physio, bio mechanics coach, PT qualified in dynamic assessment) to see what you need to work on. I would say stick with overhead squats as they will help a lot with squats and general full body flexibility.