Every 2 to 3 yrs. With an exception no first yr models. Paid the price in 05 with the RT did the same again in 08 with the XP. This time waited a year and have an XMX on the way.
I hear ya on the RT, i got talked into one in 05. Was riding an 04 900 Cat, once on the RT i sure missed my Cat.....WAY better sled but we did work out the bugs and it became a real good sled, far better then the Cat but not at first. It took 2 seasons of working on it though.
I switch mine out whenever I get a permision slip from the other half, so far about every 3-4 years.