1) You could go 3 weeks before your first carb up, 2 might be better.
2) No, carb up every week. You need to restore leptin levels in order to get the maximum fat burning effects. Going too long without a carb up will hurt your thyroid and fat loss will come to a halt. For less of a chance of fat spill over, just do a 1 day a week carb up.
I would say initially you can go for a carb-up after 20 days/2 day carb-up. Later on you can carb-up every 5-10 days depending on your thyroid activity and heat levels. Meaning if your temperature falls too low it's a sign you're not carbing up frequently enough.