The "tune" doesn't need to be redone, but if the shock won't hold nitrogen pressure, it will need to be rebuilt (seals go bad).
I'll have to ask
@pcoplin how often I should expect to fill them. I suspect some of it has to do with how hard you beat on them. However, the shocks are super easy to check with a with a gauge that goes above 200 psi. You just hook it up to the schrader valve. The shocks should remain at 200 psi. If they lose charge over the period of a few days, it means they likely need to be rebuilt.
This is taken from the Fox Owner's Manual:
Fox shocks should be charged with nitrogen only. Nitrogen will not hold moisture, and expands less as temperatures increase. Do not fill your shocks with compressed air because this will introduce moisture to the shock causing internal corrosion.
FoX shocks should be left at the factory setting of 200 psi of nitrogen. The nitrogen charge in the shock is to pressurize the internal floating piston (IFP) and avoid cavitation. Tuning should be performed through spring rates or shock valving changes and not with changes in nitrogen pressure.
We recommend checking the nitrogen pressure regularly. We also advise cleaning the bodies and shafts several times a year.
It's worth checking out the Fox offroad shock owner's manual here:
https://www.ridefox.com/fox17/dl/offroad/605-00-035_REV_B.pdf