Most vehicle manufacturers recommend changing the transfer case fluid every 30,000 to 40,000 miles and sometimes sooner especially if you do a lot of towing with the vehicle or use the 4 wheel drive option more often.
As long as you change the transfer case fluid every 30,000 miles to 40,000 miles that should be good enough as the transfer case fluid doesn't need changed as often as the engine oil or transmission fluid.
I always change the transfer case fluid at least every 30,000 miles as preventative maintenance and it will help keep the transfer case working properly for years to come and for many miles.
Keep check on your vehicles fluids and care for the vehicle and it will last a long time.
I have a Chevy Truck that I have maintained and it's a 1992 Chevy Silverado 1500 and it's got 500,000 miles on it with the original transmission and engine and it's still going strong.
I'm not the only owner of the truck and didn't put all that many miles on it myself but I did put 200,000 of those miles on the truck and I've always taken care of the vehicle.