I always leave a CO2 in the gun, sometimes for many months on end and while I respect that others disagree, this is for me the best practice for longevity. I've never had to replace a seal.

What damages seals the most is the expansion of the seal when the bulb is unscrewed, combined with the contraction and cooling of the escaping CO2.

Keep the seal moist and lubricated and be sensible and you should never have to replace them. As for leaving a bulb in, it makes no difference to the life of the seal. If you want to empty it then either shoot the gun empty or if possible dump the gas by manually opening the valve (keep the gun pointed in safe direction when doing this, if the gas is contained within the magazined rather than the gun, watch your fingers). Always ensure a gun you put away with gas in it is clear.