ive been shredding wrong this whole time... now when i alternate pick correctly i can only go 16th notes at 100bpm... how long do you think it will take me to get back to 208bpm?
Yeah, I had the same problem once. Cleaning up my alternate picking technique to decent level - 16th notes @ 120-130 bpm - took about 6 months and from there on pushing the speed barrier was fairly easy. Just remember to play slow and stay patient.
I don't know. Not enough info, like how long you were out of action, and like FP said how wrong the old technique was. Things that will positively affect how long it takes:
1) How you think about your goal. Substitute "I want to play at 208bpm", with "I want my new good technique to be really robust and locked in, with no temptation to return to my old bad technique". 208 (and higher) will happen by itself, and feel effortless, when you have achieved this goal.
2) Quality of your practice. The more focus and self discipline you apply to your practice the faster you will get there.

Good luck with this!