"how long should plaster last" is virtually impossible to say. There are numerous factors that play into that. Here's a few so you can see I'm not just being a jerk with my answer, I'm being honest and upfront.
How the plaster was mixed, what was mixed in with the plaster, the length of time it was mixed, and the water content of the plaster mix all play into this answer...
Then you get into how the plaster was troweled, etc.
Finally you have water chemistry.
I've seen plaster jobs that have lasted 20-30yrs with zero cracking/pitting. I've heard stories of them crapping out in under 10. (IMO, if the chemistry has been taken care of a good quality plaster should last >10yrs). If it lasts less than that I'd bet a dollar there's a reason and it's in materials and/or workmanship.
Regarding cracks specifically I've heard stories of pool plaster cracking if they were drained and left that way for an extended period (Although I have personal experience that 100% contradicts this). Another possibility is the crack goes into the shell as well. If this is the case, that's bad.
Other more knowledgeable people than I will be here soon I'm sure. These are just my experiences, and the opinions I have based on those.