They will come when enough pixel people have died for the reinforcements to arrive without crashing your computer.

I had the same issue recently, the ticked box not allowing unlimited men. The solution for me was to go into the preferences file and manually change the "Unlimited men" to TRUE. For some reason that hadnt happened when I ticked the box. So dont assume, go check :)

My understanding is that there is no random, will they come?, issue. Its all about computer performance specs. Unless you are playing with BI and are fighting night battles of course.