I've been using 2% HQ for more than 3 yrs now, and now my skin has stopped benefetting from HQ, and has become extremely sensitive to the sun, also, it has gotten quite darker and redder and thin again... My question is, if I stop using HQ, how much time will it take for my skin to get normal again, and recover from the damages caused by HQ? And these effects aren'y permanent by any chance, right?

You will get very dark after hq use.You have got immune to it the hq,that why it not working no more for you.