My NX kit takes a while to warm up, I'd say about 10 minutes or more to get to 1050 PSI depending how much is in the bottle.
If you get a bottle blanket it will speed the process up a little, since it'll retain the heat and disperse it around the bottle further.
BTW your "20 lb" bottle is a 10... they weigh around 14.5 lbs empty and 24.5 lbs full, so there's 10 lbs of nitrous.