the rule is to go from 40-140 in 4 hours or under.
with whole cuts of meat like butts or brisket you can kinda sorta get away with more than 4 hours as long as you don't inject or poke holes in them. but since a turkey or a chicken has a huge cavity in it you need to make sure that you get it to 140 as quickly as possible. so make sure you let it thaw completely. 20# is a lot of bird. you will want to go as hot as you can manage and still keep a consistent temp.
i think most people would recommend spatchcocking a bird that big. i usually try to leave my turkeys intact (mainly because it just looks good when your slicing at the table) but a bird that big you will probably want to break down a little bit first just to make sure you are safe and aren't cooking it all day long.