500N- it's an investment in the future. Seedcorn guys have been told to expect similar timeframes. By the time training on type is complete there is then still a useful period of return of service. They're not seen as one off 'exchange' type tours.
Eventually I would expect Lt Collins to get an F-35 slot at Eglin (which is the point of keeping a group of fixed wing trained FAA pilots), hence extending his time in the US even further.
There are RN and RAF engineers who have been working on F-35 in the US for nearly 5 years now.