Onion skin weathering is an ongoing process. How long it would take for a specific rock to completely weather would depend on its size.
It takes a very long time. Many years in fact.
From five minutes to millions of years.
Onion skin weathering is a Geological process that happens mainly in deserts. As the rock heats up and expands by day, and cools and contracts by night, stress is often exerted on the outer layers....
Biological weathering can take months or even years! GO TWGGS science project !
30 minutes or a half an hour.