I would change the first fill in not more than a year.
Why? The fluid is most likely OK, but initial wear in particulate matter is what I am concerned with.
I am looking for more life than 100,000 miles out of a car. Mfrs. are often not shooting higher than that.
Keep abreast of what is the newest and best suitable fluid for your car.
Amsoil and Redline make superb replacement manual trans fluids.
After the initial change, you can go years with good synthetic fluid.
How long? I change fluids too often. You can probably leave a good premium fluid in for another 100,000 or more, but 50,000 changes are going to keep things in peak condition.