Degass a PCP ?

So my Benjamin discover is coming in the mail sooo excited to get my first ever PCP :) i have a question tho 
how long can Air be stored in the gun ? so i need to like shoot it down to 0 PSI every time im done shooting for the day ?can i use the bleed on the pump to let all of the air out ?
or are you supposed to let all of the air our or are you supposed to leave air in ? im a complete new guy to this been watching Tedsholdover for 3 years but iv never thought about keeping or not keeping air in the gun ? just wondering because sometimes i may only get 1 shot per day at a squrrile so if so if i have to shoot it down thats a lotta pellets or can you dry fire it down ?