Well, if your boa is healthy, it can go 20 days without eating. As far as feeding right before you go, you'll come home to a nice stank in your house from the defecation left in the cage, and your boa will be living in it as well until you get back. That could be a matter of stool in one corner of the cage, or stool smeared all around, just depends! Not the most sanitary conditions to leave your animal in. You may want to find someone you can trust to atleast clean the cage while your gone. What happens if the first day you leave, your snake spills the water bowl??? Twenty days is also a good while to go without being there to be sure none of your heating elements(whatever type you use) have blown out, overheated, shorted, blown a fuse, etc.) stop working, especially this time of year. Not sure what part of the country your in, but power failures do occur. If you live in the North, and you lose power for a couple days while your gone, the result would not be good, and could mean a dead pet. There are many considerations. The least of which is if it can go 20 days without food. That is a minimal concern for a healthy boa. Hope this helps a little, take care!
Paul