Hi
It's as realistic as you want to make it. As long as you have the hours, (around 300 P1 on a PPL), the cross country time, relevant medical, (class 1 OR class 2 for a flight instructor), and all other things in place, it is perfectly feasible.
I may be wrong, but I don't think the ATPL exams expire for issue of an FI rating, (as you know, they do for CPL/IR issue).