It could take a few months, or a year. Snakes grow at their own rates.

Also, a 55g is WAY too big for even a large BP. They like smaller enclosures, even as adults, so the tank you'd need for an adult would be a 30g breeder or a 40g at most...and make sure the tank is wide and low regardless of size.

ZooMed UTH pads are great, but you NEED a thermostat to control any heat source you use, since they can get up over 120 if just plugged into the wall. Depending on budget, you can get a ZooMed ReptiTemp for $21 for 'in the meantime' use, or the good stats, which are Johnson($80), Ranco($75-80), Herpstat($100+), and Helix($110+). You can even use a basic lamp dimmer until you can get a good thermostat.