I take it you're talking about the RE LAL course for those contemplating life as a Sapper? If so, there is no specific run time on the LAL. However, my advice is that you ought to be confident of cracking around 13:15.
Definitely put the effort in - but I'd perhaps take issue with the idea of there being no time limit. If you're aspiring to join the Army, you should be able to achieve the minimum standards laid down by the Army - ie running 1.5 miles in 10 min 30 sec. This is very very easy, and is a bit of a combat indicator that you're serious about joining the army, and have put a bit of effort in to preparing for Army life.
Once you've made the committment to join, try your best to get this down to 9 min 30 sec (at the slowest), and make sure you can do at least 60 press ups and 60 sit ups - again, a little bit of phys each day and this isn't hard to do. That way, when you get to training, (a) you'll have a bit of a head-start on the guys that haven't bothered, and (b) you'll show your DS that you've got the motivation to do some phys in your own time. There's a lot you can tell about someone by their level of fitness - if they've got the motivation to do phys in their own time, they've probably also go the motivation to be a decent soldier.