When app get rating / review?
This depends entirely on the app.


Some apps promote reviewing and get more reviews.
Some apps are designed for kids and do not get as many reviews, but sometimes get more reviews due to parents reviewing the app.
Some apps' market demographics means that the users are unlikely to review an app.


There's no single average time frame. It is entirely up to the user whether they review an app, although you can mention/promote reviewing in the app itself if you wish to expedite the process.