I always leave them in on every gun I own, I've never replaced a seal.
I have 22 CO2 Pistols, some well into their second decade of use, I had more and some older ones but some were sold on and my Original Umarex PPK/S recently gave up due to metal fatigue in the frame, the seal was still fine and spent most of it's life with a CO2 bulb pressed up against it.
I am sure that most of the CO2 eventually will leak out, but my view is that constantly removing CO2 does far more damage to the seal that leaving it in. This is far worse when there is even a small amount of gas left in the bulb, removing the bulb releases the pressure making the seal expand, the escaping gas freezes the seal causing the seal to try and contract because of the lower temperature. Mechanical expansion and contraction due to temperature drop happening at the same time is what messes seals up.
Others may disagree, these are my observations of the many guns I have owned since CO2 came off ticket.
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