Depends on the bike, my weight, wind, solo or not and the terrain.

Road bike on my own doing flats about 16-18 mph no winds... with a head wind half way about 15-16.
Road bike with a group about 18-20 but not for a full 50 miles.
Road bike with hills and wind about 15-16.
Mountain bike no matter the conditions between 12-14 in the woods. If I do the open area which is 6 miles around an old cranberry bog (Bog 18 google it) its about 12 miles with wind, and 14 without.

Most of these are ending average speeds by my Garmin.