Well, the creature they are based off of has a CR. The summoned creature's CR is with the one that is able to take 6 seconds to call that treant on your head.
From previous threads on this topic, an important idea was that you can 'defeat' someone without actually murder/death/handcuffing them, but rather by forcing them to escape.
I found myself questioning how you would deal with the XP of a guerrilla summoner, who spammed a couple summon spells, hid themselves, and retreated if their meatshields died in order to mount another, similar attack. Traditionally, I do believe you would only get XP for beating them once, no matter how many hound archons they sic on your rear.
In such a case though, where the summon monster is presented as pretty much a stand alone encounter, I might advise you to treat it like it was a normal monster, since the players would feel cheated otherwise. I mean, does it matter whether that T-rex was real or not, when killing it was just as hard either way, and you were just as likely to die?
Admittedly, having an enemy that literally cheats the players over is a good way to get genuine animosity towards it. Make sure he picks their pockets too, since that is another good way to have them tearing the entire kingdom apart for one guy.
