My Lands: Black Gem Hunting

I am new to game, my first hero is guru and I see he can learn the spell "Mobilisation" and summon troops. My questios: how long does it last ? or are the troops mine to keep until killled ? Also it lists various numbers of various types do we have a choice of what it summons ?

Hi,
you summon random number of troops, quantity depending on skill level, and they are yours to keep and feed till they are killed.