| Hermano |
This is a common quesiton also in d&d but the common idea is that either the called must be able to complete the task on its own (MR. Meeseeks!) or it should have a limited duration.
The caster has to negotiate the agreement with the called and any tasks lasting longer then normal 1-20 days should be prohibitively expensive if at all possible. You could have the caster tricking the called - but Planar ALLY is actually the caster begging for help from their Diety who could be upset if it was abused. The called putting its life on the line would turn down any offer not worth it or made by faithfuls who shared their dogma.
It didn't come up for us as we skipped this dungeon entirely since I had swapped the plot devices of AP 3 and 4. Casandalees AI core was trapped in the choking tower calling out for help to nearby timeworn equipment and establishing a dialogue with the players. The next AP was about returning her to her body.