Without question, the prolonged use of hydroquinone will cause irreversible hyper-pigmentation/excess darkening of the skin. The worst part of the onset of this condition is that it is so gradual over the years, you won't notice it until it's too late. By too late I mean that no amount of hydroquinone will ever be able to re-lighten your skin. And any amount that may cause moderate lightening could only be at extremely toxic, cancer causing levels. Another affect of this condition is that even once you discontinue its use your skin will get progressively darker as time goes on. This is simply because all people generally darken as they age and as they age, are consistently exposed to sun. This natural darkening and aging process of time is simply heightened by the condition of hyper-pigmentation. The first warning signs of excessive and irreversible skin darkening is that you will need twice as much for the same results than needed at earlier applications. The body/skin build a resistance to hydroquinone, which essentially is the greater production and store of melanin in the skin.
It's better to use natural products, PhamaClinix products etc, that don't use Hydroquinone when trying to eliminate blemishes that may return anyway. Its also important to remember that over time most if not all blemishes and acne fade away and that it may be better to wait that process out.
For those who wish to use a skin lightening product for a "better" complexion, remember this feeling is a socially induced thought, that lighter is better. As corny as it is, true beauty is about the person inside the face, the mind. Don't get caught up in the trap of trying to please people based on what your supposed to look like as if that look actually makes you a better person. There is no amount of vanity worth the irreversible damage and destruction of your face. Love it for what it is.
I have seen the results of hyper-pigmentation with my own eyes. It's very serious and real.