@plainoldme,
It's not a specific amount of time usually. And I don't believe it's called a trial period.
Just, you are hired as an assistant professor (in the field I'm familiar with). This is a position without tenure. It is understood that you are going for tenure, but that it won't happen immediately. ("Tenure-track position.")
Then you are an associate professor -- this is progress but still not tenure.
Tenure is when you're a full professor.
All of these stages are recommend by other faculty and voted upon by the faculty as the first step in a process that goes through the dean and ends somewhere slightly higher up (again in my experience). The faculty votes are most important. It's rare that higher-ups question it, though it happens.
The recommendations can happen at any time. They're goosed along by things like a) getting an offer at another institution (they're trying to keep you) or b) superlative work.
I think there is an outer limit by which you should either have tenure or know you're not going to get it and make decisions accordingly (the person I'm familiar with got tenure very early so I'm less familiar with the other side of it).
You might also be thinking of a post-doc position -- that has a built-in time limit of usually three or four years. The other positions don't have time limits, you just are one until you are not. We know people who have been associate professors (without ever getting tenure, but sticking around) for 30+ years.