I replaced the fluid in my 1996 T5, at about 120,000 miles, not knowing the previous owners service history. edit: I'm about 145,000 miles now, with no major issues.
. don't have the same clutch packs that are in autos. Just gear on gear contact.
The story goes that as the clutch packs wear down in autos, the clutch particle end up in the fluid, and the particle laden fluid helps compensate for that wear, keeping the clutch packs working correctly. New fluid removes those particles and the clutch packs fail to operate properly. I've only heard of this anecdotally, and of transmission failing as soon as weeks later.
I've never heard of the same thing happening in manual transmissions. Though the oil can break down through normal means. And it wouldn't hurt to get out any metal shaving from grinding missed gears.
The scheduled maintenance manual from my car lists an change for the auto transmission every 30,000 miles, about the only mention for the manual that is in there is "Check manual transmission fluid.".
every year; nothing about intervals to actually change. I just used standard off the shelf "Mercon V". Probably the same for yours, just doublecheck.