how long does it usually take for back pain to go away?

Off topic but I thought it was going to be about something else after seeing that title and poster

If you might have hit it hard enough to break something, you should go get it checked out because a fracture there is dangerous.

If you think it's just a bruise, it'll go away in the same timeframe as any other bruise, i.e. a few days.

Hopefully the pain will have significantly reduced within 3-5 days if you are lucky and able to avoid all sources of strenuous exercise.
You can try rubbing arnica cream or nervex pain relief cream twice a day over the injured part of your spine and areas surrounding it, this usually works well for me.

Sometimes slight changes of diet or a relaxation of calorie restrictions can help the body to heal.
Coworkers with brittle bones have told me that adding more citrus fruits, dairy and oily fish into their meals when suffering from painful bruising or spine injuries has been very helpful for them & aided their recoveries.
I'v never tried it, I only eat dairy a few times a year and won't touch any seafood nor citrus fruits.
Might work for you though.
Good luck!

Normal back pain - a couple of days
A bruised back - a week
A damaged back - a couple of weeks
A herniated disc - roughly six weeks
A broken back - several months

Won’t know until you get your back examined by a doctor. It could just be bruised or you could have hurt/broken something.

i hit my back really hard last night. sort of right where the mid part of my spine is. it's pretty bruised and hurts like a mf, barely got any sleep last night :< can't imagine days of losing my beauty sleep ever night for days, is it likely to go away soon?

Sorry to hear about your back! Back pain can vary. It might stick around for a while if it's a nasty bruise and hurts a lot. But seeing a massage therapist could help it heal faster. I had an arm injury once, and going to a therapist scarborough helped me out. They eased the pain and got my arm moving better again. If your back doesn't start feeling better soon, seeing a doctor is a good idea.