Paul,
It all depends on the type of approach you make. If you are making a visual approach then there is no need to go much above 2 miles past before making a descending turn to line up with the runway a couple of miles out. If you are flying an ILS then a little further, but not much, probably 4-5 miles minimum. If you are flying a non-precision approach then you will probably go further still. This all depends on where you are and the surrounding terrain, for instance at Amsterdam for an ILS approach they will feed you in quite happily at 5 miles or just less, whereas if you are at Innsbruck you will go over 20 miles out before joining the approach.
Guidelines for descent vs altitude, we generally use 3 time table, so from 30000' we multiply the number of thousands of feet by 3, in this case 90, thats how many miles we roughly need. Having said that we do have ways of reducing that if necessary.