Mine had 70k on it when it went.
When I got the car it had 63k on (IIRC) and it had no unusual noises or wobbing gear lever.
I discovered cruise control when I was going to Anglesey every weekend and I used to have cruise Control on for about 100 miles each way on the A55 and M56 always in 6th gear often just using the speed control on the stalk rather than the pedals. My box ate itself within 7k miles so thats my basis for saying don't use cruise control, I might be wrong about it, but dragging the car along like that can't have done it any good.
Jasons, For your box to have died again after 15k the repair was done badly, The same thing happened to me. First time I had it fixed the repair centre offered to do it right, or do it cheap (with no warranty). I chose cheap as I figured if they were replacing the broken bits how bad could it be?
Turns out they only replaced the bearings and not the smashed cogs or the bearing seat in the end casing, To be fair they did tell me the car might not make it round the block and the best thing would be to sell it asap. It lasted 6 months then smashed itself to pieces again.
Second time round I had it fixed properly (cost me 4 times as much) but now 20k on it's still going strong.
(Also if you drive it quite hard you'r probably not doing it an harm, From what I've read it's the tourqe that kills these boxes, and ragging it through the gears isn;t going to get maximum load through the 6th gear shaft like sitting on the motorway in 6th does)