I have been playing drums for about 2 years and I have used my legs for double bass. I got up to about 140bpm but I haven't been able to go really any faster for the last about 9 months. I looked at leg speed online and the fastest I have ever seen someone go with only legs is about 180-190 bpm. So since I have seen much faster with just ankles I decided to switch to them. So I have 2 questions. 1. I can do about 145bpm 8th notes on my right ankle but only 80-90 bpm 8th notes on my left ankle, is this normal? 2. How long should I work on this every day? Is there a point where I could overwork my ankles?
Firstly, when actually playing drums, pure ankle motion (note: pure ankle motion is the same motion as heel down, but with the foot held in the heel up position, like dribbling a basketball at a low height) only comes into play around 230 bpm. It's basically almost impossible to play 16ths at 150-170 bpm while doing ankle motion with the heels up with any amount of control. Before that (180-210), it's a mix of leg and ankle motion with the ankle becoming more prominent the faster you get. So, I would say you really ought to practice both legs and ankles, legs through heel up playing and ankles through heel down-- I learned ankle motion just from doing single stroke rolls on the floor without pedals, going from 120 to 180 bpm. If you can do heel up leg motion 16ths at 200 bpm for 5 minutes, and heel down 16ths at 180 bpm for 5 minutes, then you have a pretty strong base from which to approach ankle playing, which guys like David Haley, Inferno, Alex Micklewright, and Flint Marshall use to rip 16ths at 270-280 bpm.

If you can't progress in speed with the legs, then there's nothing wrong with you, you just need to practice differently (or if you just noodle around on the drums aimlessly, start practicing). Doing double bass for long periods of time (for consistency and control, infinitely more important than speed) continuously will do the trick. Just play 16th notes for 5 minutes straight with a metronome at 140, do that again the next day, and then raise the tempo by 5 bpm, and try to practice at that tempo every day until it feels comfortable, then just repeat until you eventually reach your desired speeds. Learn to mix the two motions also, at 180-220 bpm you should be kinda bouncing your ankle off the pedal but keeping it in place with downward motion from the leg.