It depends on how long the script and movie is intended to be. If you are shooting a feature length 90min+ film, then 20hours isn't a lot at all. If you're shooting a five minute film, then 20 hours is a lot of film to use. So to asnwer your question, it is a subjective question, that is dependent on the intended length of the movie you're shooting.
Peter Jackson shot over a hundred hours of footage on his 3 hour king kong.
