Compulses last about 30 minutes (can't remember the exact time).
At the end of that time they are released and disappear, unless they have been killed first or you have transferred them.
If they are released due to time then you do not get any experience from them.
Compulses will not attack you unless you attack them 1st. Note that SH2, B2 and C2 will hit your compulses and then they will attack you. You can stop their attacks by typing "Stop". Most of the normal GC commands work on compulses.
Higher compusion power only affects the level of NPC you can compulse, not the time compulsed or the power of the compulsed NPC.
There is no limit that I am aware of on how many NPCs you can compulse. However, if they are gargs then do not expect to be able to read any messages as the garg spam will fill the screen.