It varies greatly by model and builder. If you want to get an idea though, go to towerhobbies.com and look at the kits on the site and see the what engines they put on each. You can get an idea that way. Most kits have a range of engines, and most use the biggest engine in the range as a starting point. Many people now use bigger engines than they recommend to do more vertical manuevering. Its not the end all, but it will give you a starting point.