It's up to you. The manual transmission is meant to be sealed and you won't lose transmission fluid. The synchros do wear and so do the gears when running in (and when shifting hard), so there will be a build up of metal particles. I'd do mine every 2 years or after every track day. If I drive hard, then every year if I felt like it.
For your auto, it's better to change it every 2 years.

