Depends on what kind of contribution you want to make & how much you want to learn, and what skills you already have.
If you wanted to be on the cutting edge as a fusion physicist, and let's imagine you are a bit rusty on calculus & physics, you'd probably be looking at >2yrs remedial math/physics classes at the undergrad level, plus about 6yrs of graduate work. If you're able to get into a big-name program, you could be doing something useful after the first couple years of the graduate program.
If you're a data analyst/engineering or programmer, you could probably get involved in a less direct way (supporting the science team) much faster.
Here's a pretty good list of (primarily US-based) jobs in the fusion sector, including both scientist & other roles. Have a look at the qualifications these roles require.
https://usfusionenergy.org/opportunities