If you struggle with pigmentation it can be very unsightly & distressing for you. I completely understand this & you probably want to brighten pigmentation as quickly as possible.
It takes about 18 months to 2 years to effectively brighten pigmentation and make sure it doesn’t come back. The amount of time varies according to your skin tone, how deep in your skin the pigmentation is and what caused your pigmentation in the first place.
Let me explain…
Ladies who have black, brown or olive skin tones tend to get pigmentation and an uneven skin tone. This is because our skin is so good at making pigment/melanin. So the same melanin that gives our skin colour also causes pigmentation.
Melanin protects your skin from the harsh rays of the sun, that's why when you're in the sun for a long time your skin gets darker. A dark skin tone is so good at making melanin that any form of inflammation, a small cut even, can trigger your skin to make more pigment. Once the melanocytes are triggered to be hyperactive in a certain area, they become hyperactive all the time.
This means that when brightening pigmentation on black or brown skin, you cannot be aggressive or use irritating products, which will create more inflammation, as you will risk creating more pigmentation. 
So you can't have chemical peels every week, because they create inflammation for example. Rather have these bi-weekly or once a month. If you want to try treatments like microneedling, you have to make sure that there is no inflammation on the skin & that you space the sessions accordingly otherwise you risk creating more pigmentation.
Sunscreen is important in preventing more pigmentation, even when you're inside. You're wasting your money if you're trying to brighten pigmentation without sunscreen.
Mineral/physical sunscreens don't create inflammation in the skin but most of them leave a grey/white cast on dark skin. My favourite at the moment that won't leave a white cast on your skin is Dermalogica's Invisible Physical Defense spf30. Chemical sunscreens go up to SPF50 but can cause inflammation in your skin
Another ingredient that won't cause inflammation that I love is Alpha Arbutin, you'll find this in Skin Functional's Dark Mark Management
Brighten Pigmentation - How deep the pigmentation is in your skin
Our skin has two layers, the epidermis and the dermis. Pigmentation can lie in the epidermis or the dermis. Most products work in the epidermal layer of the skin so if your pigmentation lies there, it can brighten pigmentation effectively.
The dermal layer is where your blood vessels are and skincare products don't penetrate to that layer. If your pigmentation lies in the dermis, it becomes really difficult to treat it. Dermal pigmentation is difficult and takes time to treat. This requires patience
Besides the sun, other things can cause inflammation in your skin resulting in pigmentation. You have to determine what has caused your pigmentation and treat that first before you start to brighten pigmentation.
I have another blog post called Is Your Face Darker Than Your Body that covers all the different skin types, skin conditions and environmental factors that can cause pigmentation.
As much as pigmentation is unsightly, it requires a lot of patience to brighten it. Harsh invasive treatments that cause inflammation will only create more pigmentation, especially in black or brown skin.
Patience...