Yo, Adaptability isn't the main thing you should be looking at. Check how much it raises Agility (it's also affected by Attunement slightly).
For "Fast-Roll" (under 70% burden), these are the breakpoints:
85 AGL = 8 i-frames.
90 AGL = 9 i-frames.
95 AGL = 10 i-frames.
100 AGL = 12 i-frames.
105 AGL = 13 i-frames.
110 AGL = 13 i-frames.
115 AGL = 15 i-frames.
120 AGL = 16 i-frames.
Remember that rolling distance is not affected by agility, though. It's affected by your
equipment burden and seems to scale pretty fluidly with it (i.e. no breakpoints, just a steady ratio). Less equipment burden = More distance covered per roll.
Source:
http://darksouls2.wikidot.com/agility (though don't browse around here unless you want to get spoiled).
EDIT: Note that you get 2 i-frames between 95 and 100 AGL, but you don't go over 13 until 115 AGL. This is sort of a point of diminishing returns. Also, in order to get max AGL you need to max out both Adaptability and Attunement, so you'd probably get better benefits elsewhere by leveling other stuff (more attack or spell damage, for example).