Wilts,
Another point of view if I may (I have not been in as long as you (from what I gather) but I have been in a while (mid 80's) and I have formed some opinions on this).
I would not discourage a "how long?" candidate - in fact I think it shows a high degree of common sense. I was a '"how long?" guy before I joined because I didn't KNOW what it was like. Many of my colleagues did their 2 tours (i.e. the armed forces got their 'money's worth' from them) and a lot more stayed in (inc me).
If I had a candidate who gave me the "I want to serve my entire career in the forces" line, then the reply to "how do you know, you are not actually doing it yet?" was often quite.... delayed.
There are 2 sides to every story.....
MT
ps) However, I must say that I have had a blast my entire time in and I'm still loving it :-) Nothing outside compares (and I did some stuff outside too!)