Way too many variables.
I've had contract where I've known I'd be there for the next year, and some I found out a couple weeks before I signed off. Sometimes people only find out after they've signed off and have gone home. Usually the more 'skilled' or specific the job is (i.e. marine, entertainment, management), the earlier you know your assignment. The less skilled or more easily fillable position have a lot more flexibility in assignments.
My line (RCI) we have a few days for the most part to decide . . . but then the offer can come anywhere from a few months to a couple days. And for our line, we need to let the office know 1 month out if we're not coming back (if we want to be considered for rehire). But then, for some jobs that have 'wanted' skills - I've know those fired, put on 'double secret no-rehire status' and 'no shows' to be rehired.
Are you referring to a specific situation or crew member? Any way of asking that person directly?