Depends on many things.
-size of the clay bar
- the amount of contamination on the ar itself
- what you use as lube(some people say that using purpose built clay lube enhances its life. E.g- CG luber and dodo born slippy clay lube.)
-how big your car is
- how clumsy you are:P( if you drop it you have to discard the whole piece)
I think the 200 gram clay bar should go well for 4-5 sedans. Youll have to buy detailer spray after 1.5 sessions:)