GBBR is always cocked and ready to go.
OP: not supposed to food feed you, but in general, if you're using green gas mags, rather than CO2, you can probably get off 30 to 60 shots off with a mag. Assuming you're talking about GBBRs that accept the stanag mags.
There's several different gas systems out there, too.
I run a KJW setup as my primary, and it has the thermold mags, which are supposed to hold more gas. I usually refill my gas every 2 reloads.
I have both a WE open and a closed bolt as my backup, which I rarely use, as well as a KWA and WA, which I've never actually used in a game, and I gas up those mags every time I reload my BBs.
GBBR stanag style mags are generally real cap, and it's a good idea to keep them full of gas. If you're going to reload your BBs, you may as well refill your gas.
As for the charging handle: The GBBRs that you'll find on the market in Canada, have a moving bolt, that functions in the same way the real steal guns do. So you have to charge it in order to load a BB into the hopup/chamber, as well as cock it.