But how do you define "full"? How do you know they are full? I know TM mags start to spill gas. But what about other brands?

To Pestobanana: Because if you fill a mag for 6sec or more you can feel that the fill valve gets really cold.

It shouldn't get really cold unless something is leaking.
Fill until you stop hearing propane moving through the valve, that's how you can tell it's full.

I see. So how much time do you think it takes, approximately?

Also, what do you guys put in your TM mags these days? Do you just put propane? Or duster? Is it tetra- or di-?

Maybe 5 seconds or so? I'm not sure. As for tm mags I use propane. Are you askin these questions because you use tm pistols? Their fill valves do not have o-rings you can freeze.

I do have a few TM pistols but never filled them with anything yet. Just don't want to screw them up. My secondary is KJW M9 with 3 mags. I think I overcharged at least one of them because it started leaking recently. If I fill the M9 mags for 5 sec it is not enough to empty the full clip, so I usually charged them for 6-7 sec.
I also have WE Scar with 5 mags. I bought them all used and had to reseal all of them and oil the valves. I think they still leak a bit but not as much as before. It takes a couple of weeks now for all the gas inside to leak out. So basically, I just want to make sure I don't screw them up.
I am wondering what other people experiences are like.

Gas mags leak all on their own sooner or later, its inevitable.

You aren't going to cause a mag to leak by "over filling"
Like I said, fill until you stop hearing the sound of filling...

If it has not been said, I like too stick my ear too the tank while filling my mags.
When you stop hearing flow, Its full.

I store all my magazines in ziploc freezer bags to keep the seals from drying out.

I haven't tried this.. Can anyone else confirm this that it works?

It helps detecting leaks from what I know (ie. if your ziploc bag inflates, you have a leak)

It says not advised for use in cold seasons but what is his definition of cold?what temp is too cold?what temp is pushing it but will b ok with proper care?i wanna play a skirmish on nov 9. night game......we ak74un we g17 using propane

I've seen people state that propane starts to lose pressure between 5-15c.
An overnight game in Southern Ontario will be pretty close to 0, according to weather forecasts for the 9th, so you may want to have an AEG backup.

If you keep your mags inside your coat, to keep them warm, you may have decent results.