I'm gigging with my band outside in January. It's in a tent, but its open at the front.
Now I have some questions, will it be a problem for my Vox AC30's tubes? I'll turn it on a half an hour before we start.. Is it a bad idea? (I live in Belgium, so it will probably be freezing )
Well it's bad in the sense that they'll die slightly sooner than they would if you didn't do that. But as long as your tubes aren't on the brink of dying, there should be no issues.

That amp has a standby, right?
Spin 'round carousel when your horse isn't screwed in.

My band:
Fractured Instinct
(For fans of Death/Groove/Prog Metal)

Ibanez RGA42E
Ibanez S420
LTD H-301
Ibanez RG520
Peavey Predator USA
Douglas Grendel 725
Line 6 Pod HD500X
5-10 minutes should be more than adequate, but really it up to you, it's not going to do your amp any harm if you leave it on for longer than that.

It depends what kind of conditions you keep the amp in too. If it's sitting in the cold for a long period of time then you'll probably want to give it a little extra time, but if it's being transported from a heated room, to a heated car then played 20-30 minutes later it's really not a big deal.
I have Used My Blackheart Handsome devil at an outdoor Hunting party, just 5 days ago. Damn cold here in Kansas, 28 F that evening, I had no problems, and it doesn't have a Standby switch.

I think you will be fine!

PRS Santana SE
Ibanez MBM2 EMGs
Fender MIM Roadhouse Strat
Ibanez AS93 BLS

Carvin V3m
Blackheart 4x12 cab

Electro-Harmonix Metal Muff
Zakk Wylde Wah
JamMan looper delay