Thanks for all the advice guys!
I did an extensive test last week with the new recoil spring, again standard Glock 17 lbs on a plastic guide rod, and everything worked perfectly. Temperatures were again sub zero and my G20 fed 100% in the +100 rounds I tried. Each time I loaded a full 15-round mag, fired 5 rounds and reloaded the mag to its full capacity again. I did this because with the worn recoil spring the feed problems occured in the first couple of rounds.
So I guess if you shoot real 10mm rounds with the original recoil spring, you should change it between 3000 and 5000 rounds. My G20 had about 4400 rounds through it, and the worn recoil spring combined with the sub zero temperatures gave feeding problems. Still the worn spring didn't feel bad when I did the test described in one of the above posts, but it was enough to give feeding problems in cold weather conditions.
From now on I will change the recoil spring in my 10mm every 3000 rounds just to make sure my pistol remains reliable.