These are trucks at work...
2004 Silverado 1500 5.3L 2WD - Went a little over 415K miles before it started slipping bad, replaced with low mileage used one. I know it was definitely original because the truck was purchased new.
2004 GMC Sierra 1500 5.3L 2WD - I can't remember exactly when it was replaced, but I think around 320K miles, again with a low mileage used one. It was having some harsh/sloppy shifts, but probably could have gone a little while longer. It was most likely the original transmission, the truck was purchased with something like 80K on it.
2006 Silverado 1500 4.3L 2WD - I think the last time I drove this truck it had something like 425K miles, and I believe the transmission is original. It was also purchased with low miles (probably less than 50K)
2002 Silverado 1500 4.3L 2WD - Right around 200K miles, the truck was purchased with less than 100K about 4 years ago, so it may or may not be original.
No maintenance routine for the transmissions, they typically get a fluid change when they get a new used trans. Little to no heavy loads and infrequent light towing. They are in town delivery trucks...mostly easy, but constant driving.