The above posts are correct. There is no 'policy' but the general guidance to units is that making people do an RNFT on joining means that there is less hassle down the line in making sure people remain in date. This practice is made possible by the fact that people are now expected to be able to pass a fitness test at any time, rather than it just being a once a year event. It also has the significant advantage that anyone who doesn't pass can be put straight onto a remedial programme with a reasonable expectation of them having long enough left in the unit to either get fit or, if they don't, see the proper warning process through to its conclusion of discharge.
In my experience, getting the DOs regularly to monitor their Divisions' RNFT status and, when required, conduct / review the Warnings on time, filling in the forms properly, is as big a challenge as getting people to do the fitness tests in the first place.
Well, I don't remember anyone having a fitness test during my time in; the assumption was that once you'd attained the level required to pass out of basic training, you'd stay that way through working hard, playing hard and eating good scran - and it seemed to work well enough.
Still, I guess the work levels have changed over the years so that there's less manual work to do - maybe that's why the powers that be deem it necessary to keep tabs on the physical side of things.
Sorry if I'm "crayoning" here, but I thought it worth mentioning.
Just for people's info, Raleigh make you do one for your first PT Lesson if your there for a course of 4 weeks or more and Faslane Gym won't stamp your joining chit until you do one, which they expect you to do within 10 days. That's even if you are only doing an in house move as well!