I recommend changing it when the fluid starts to get dirty - in mine, the normal amber color begins to turn brown. The stock hydraulic system is especially susceptible to dirty fluid. Dirty fluid can cause seals in the slave and master cylinders to fail, and failed seals can contaminate fresh fluid.
I put a fair amount of miles on my Z, so I generally replace the clutch fluid at least once a year. But that's down to about every six months now that I'm on my 4th master cylinder in 73K miles
I've replaced virtually every other part of the clutch hydraulic system with great results. But there is no plug-n-play aftermarket CMC for the 370Z
