mate this question is dependant on about a squillion things... kinda like asking why is the sky blue, how long is a piece of string, and why do women go a little "crazy" from time to time with no advanced warning...
(okay, so the sky is blue due to refraction of light through water particles in the air reflecting from the oceans etc, and a piece of string is twice the length from the middle to the outside end, but no one can answer the last one... h aha ha)
After a couple of hours, or tankfulls the valves should be pretty well seated, depending on how hard the bike is run in... it is also highly dependant on what the valves are set at from factory, and the motor itself. eg my Hungten motor, i have never done the valve clearances... never had to, and it still fires quickly, has good compression etc... but my lifan 140, i keep on top of them...
i'd probably check them after the first tank full, after the third tankfull, then as required... or, if you want to look at it in hours, after the first day you take it out riding, after the second time you take it out riding, then just keep an eye and ear on them on subsequent rides...
So, check them after the first hour of riding, after the third hour of riding, then every ten hours or so of actual engine running time...