How long does it take for stretch marks to disappear?

This question is really difficult to answer. With time stretch marks can gradually become really light, almost invisible. And according to the University of Maryland Medical Center, stretch marks may vanish completely if its causes are eliminated.
Initially the marks appear as dark colored striations varying from red to brown and glossy in appearance. After some time, these marks fade gradually to become less visible within 6 to 12 months, but the texture of these scars will remain constant, similar to initial stages.

It’s always better to prevent stretch marks from appearing in first place than worrying “do stretch marks go away” later. So try to avoid sudden weight loss or weight gain because stretch marks are a physical response to excessive stretching of tissues and moderate damage to skin fibers leading to scarring of the abdomen skin.

The appearance of stretch marks can be attributed to constant contraction and then relaxation of the delicate tissue which results in the loss of flexibility.
Moisturizing skin will help you get rid of those stretch marks faster use type of lotions that contain shea butter and cocoa butter and drink more water! Rubbing Lemon Juice:Lemon possesses astringent properties due to acidic juices. They are widely used in different face wash products for clearing skin and fading marks.

This will depend on a number of factors including the severity of the stretch marks to begin with, but also the methods one uses to actively treat them.

Like any other bodily healing process, time will naturally repair & fade stretch marks over the months & years. From my experience, they will reduce about 5% in appearance every month (after pregnancy or sudden weight loss).

So you can expect around a 50–60% reduction in stretch marks over the course of the first year, when they go from fresh pinkish/purple lines to white/silver faded ones.

However the law of diminishing returns certainly applies I.e. you wont see much fading after this point. You will have to employ some stretch mark reduction treatments to either fade them further or faster.

I have spent to past 2 years experimenting with the best ways to do this.