Already slightly higher than last year, wherein 6 superhero films accounted for 15.9% of the box office. Thor will be a huge hit, so it may break 20% this year.
Yup 20% sounds like a good prediction.
Justice League is also coming out this month. I doubt it will do as well as Wonder Woman, Guardians, Spider-Man, Logan, or Thor (it might, but DC has a spotty track record so far), but it will help to push the superhero movies to above 20%.