I haven't in mind frequency but doing that oil change at all. I would say 5k miles for engine and 50k miles for gearbox.
And I am not saying what anybody should do, really.
In time of 2 years, doing gear oil change 3 times is way too frequent but yeah, you had a reason.
What's better really, 2nd hand Colt gearbox or pay for rebuilding current gearbox? Does anybody know prices of rebuilding colt/454 gearbox?
Mine is over 110k miles, but had no issues so far. Check sometimes if it's not getting too hot after motorway cruising, but always can touch it and hold hand, so possible - no any bearings dead yet.