How long does brown discharge on Depo last?

Everyone reacts differently. It's a case of waiting to see.

It can be all sorts of irregular bleeding until you've had 2 or 3 shots, so anything is possible, from no bleeding at all, to spotting or heavy bleeding. Now that you've had a shot, it's in your body & you've no real control over its symptoms.

Now that you've had 1 shot, if you don't go back for more, you may find it takes up to 18 months from the point you MISS your shot, for you to regain your fertility. It may also bring more of the irregular spotting &/or bleeding, which may be heavy.

The Manufacturer advises that you should not be on Depo Provera longterm, unless you have no other choice of contraception. If that's the case, you need to take breaks every couple of years.

This is due to the risks associated with this Drug; one of which is the thinning of your bones, until you have Osteoporosis/Brittle Bone Disease.
If you're on it a long time, they recommend you get your bones checked in an MRI scan.