Should I leave a CO2 cartridge in the gun or release it after each session?

Both schools have merit. Leaving a partial cartridge installed keeps the seals pressurized so they stay seated, but it accelerates wear on the seal and any slow leak will empty the cartridge anyway.

Most experienced CO2 shooters fire the cartridge to empty (or release it) at the end of each session, then store the gun unpressurized. If you do leave a cartridge in, plan to use it up within a week or so. Never store a CO2 gun in a hot car: temperature spikes can rupture cartridges.