My first thought was that Granny Smith meant "100 lunations" (i.e., 100 lunar cycles) when she said that the Apple Family Reunion occurs "every 100 moons."
If that is the case, the we can use the following math: 29.530589 days per lunation 100 lunations So 100 moons = 2953.0589 days There are (365+365+365+366)/4 = 365.25 days per year (roughly) So 100 moons = 2953.0589 days = 2953.0589/365.25 years = 8.085 years
However, when Granny Smith says "moons" she clearly cannot mean "lunations" since a previous reunion has a picture of Apple Bloom looking about the same age:
If moons means nights, then that's a reunion roughly every 3.3 months which is far too frequent.
It is possible that lunar cycles in Equestria are entirely different than our own. Thus, we have no idea what Granny Smith meant by "100 moons."