How long to wait before sync finished?

[interestingthings]

HI all!

I'm trying to implement a "simple small" Wahtsapp - send message function.

If it's the first time the client connects a little bit of time is needed till "all" is synced!
If it is an already known "client", less time is needed !

How can I determine if all syncs (or other pending actions) are finished and I can start sending!