Just a general guideline ...
If you are cycling from a point, and ending at the same point, (i.e. in a loop), the elevation gain will be the same as the elevation downhill. If there is headwind for half the way, there will be tailwind for the other half, so it balances out
depending on your fitness, you should finish a 100km ride in under 4 hrs (25 Km/hr) if you dont stop to rest for a lengthy period, and assuming you have no mechanical issues / punctures etc ...
obviously, if you are fitter, you will finish much sooner, but when I plan a new ride, I estimate the time based on 25 Km/hr .... if I finish sooner, it's a bonus