Android app must do something in UI thread a.k.a. main thread (at least update UI).
How long Android's application can do something in UI thread without loosing performance (no UI jittering etc.)?
In general for 60fps your magic boundary is 16ms. If you don't need more than 16ms you won't skip frames, thus, introducing no UI lag, etc.
Have a look at https://www.youtube.com/watch?v=CaMTIgxCSqU (around minute 2 the implications for developers are layed out)