On the Navy side, as an NFO (or in generic terms, a backseater) you can apply for pilot training after your first operational tour….ie you went through NFO flight school, Fleet Replacement Squadron or FRS (where you learn to fly your operational aircraft), as well as a ~3 year operational tour. So this puts most folks around the 5 year mark or so. You apply to a board, and if selected, go back, all the way through pilot flight school, back through the FRS, and back to another operational squadron. It has significant career implications, often negative ones, based on undesirable career "timing". I've known a couple, and in my opinion, it is not a really great program. Though some are more capable than others, in general, they are just too inexperienced when it comes to being a pilot for how senior they are by that point, and more often than not, they are the guys you don't want flying on your wing. I think the AF might be a bit different, because their flight training schedule is so much more efficient, thus a guy in a similar scenario isn't quite as senior when they get back to an operational unit as a newly minted pilot. For me, as a pilot, it took 2 years to get through flight training, almost another 2 years to get through the FRS, so I was an O-3 basically by the time I got to my first operational squadron. This is pretty common in the USN/USMC. NFO flight training varies a lot, from an absolute minimum in the P-3/P-8 community of somewhere south of 6 months to wings, to about a year/year and a half for EA-18G or F/A-18F. Their time in the FRS is similar to that of pilots. So very long road to a small house, on the Navy side, the time to train is mind boggling for an NFO->pilot transition.