Planar Ally summons a creature for whose service the caster then negotiates. The spell states:
If you are unable to agree on a price for the creature's service, the creature immediately returns to its home plane.
This is fairly ambiguous. Mid-negotiations an enemy could charge into the room - would the summoned creature wait around? Can one side or the other say "I'd like to think about that, I will get back to you in an hour/day"? If a summoned creature makes a counter-offer and the caster refuses to answer, or ends up mute/unconscious/dead, how long does the creature stick around?
I'm pretty confident that the answer is "as long as the DM decides" so I'm looking for both "here's some rules on this that you might have missed" and "here are some ideas/rules of thumb to help guide you on this".
Were I a summoned Pit Fiend, I'd be tempted to hold monster the caster mid-negotiation, bind and gag them, tell them I'm still considering their proposal, and then head off to cause trouble for a while.