It's hard to say. I only dealt with this once - she had left her hair braided all summer and when she took out the braids, all the hair knotted up. It took us - several people working in shifts pouring on conditioners & oils, and combing like crazy - almost a week straight to get it back to normal.

Yours, though, may be easier for several reasons. Prime reason being you only need to get the mats away from your scalp so it isn;t painful.

Personally, I'd probably start combing from the roots out towards the ends just to give your hair enough "breathing room" so it doesn;t hurt. Then I'd use twisty-ties to section your hair. Then I'd cut straight out to form the dread-lock sections.

But that's just me. I wouldn;t recommend you do anything (other than combing out from the roots to ease the pain) until several others here give their recommendations. Most here know a LOT more than I do and have had a lot more experience.

One last bit: I would not recommend the neglect method. I've seen it work wonderfully, but only on folks with very short hair (the hair tends to naturally section into small chunks and dreads beautifully & naturally as it grows long), but I've chatted with a lot of folks who started with long hair and had neglect-method disasters. Instead, I'd recommend getting it started (sectioned and help along the matting/locking process) and then letting it go on it's own from there.