Well most CRNA programs require at least 1-2 years of critical care experience (often ICU level care) so you'll need to work first. If you lucky enough to get on an ICU when you graduate you're golden. But you also have to serve time in the Navy; so schooling might not be right away depending on where your located. You might have to relocate often meaning you'll have to transfer credits and hope that schools accept them. So it could be a lot longer then 3.5 years depending on your station and if you're active duty or not.