how long do hairs turn color?

Just wondering how long does is take for the hairs to turn orange once the process starts, a friends I was looking at started and about 40 percent of them had changed. When he asked how much longer I really had no answer for him, days, weeks? No scope used to determine readiness so any tips would be nice

i hear 50% is a good number to cut at, i hear the longer you wait the more of a body high will be acheived, if its at 40% and you wanna cut at 50% id do it in a matter of days, dont let them turn black that means to waited to long