That tank would work for a full grown adult. I would wait till your BP is at least 3+ feet before moving him in there. Just make sure you clutter it up with stuff. Some people keep adults in 20L tanks and 41 qt tubs. I would say keep him in the 20gal until his hides won't fit in there, then upgrade him. All snakes are different, you can move him whenever you think it's appropriate, just leave him alone for a week to acclimatize to his new enclosure. Then, as long as you don't have any feeding issues or daytime roaming issues, you will know whether or not he accepts it...