So i've been playing for quite a while now, when I realised, I always play sitting down since i found it alot easier in the beginning.
But the time has come, i bought a guitar strap the other day, and i've been playing fine, I thought I was almost have to learn how to play again unless I kept the guitar chest height, but it's fine.

ONE PROBLEM - My back is absolutely killing me after a day of standing up playing, I realise it's probably a normal thing everyone goes through, just like when you begin playing, your fingertips hurt alot, but they toughen up after time.

So the question is - how long before i'll have no problems with back pain?
"I think the most important thing about music is the sense of escape." - Thom Yorke
You mean standing up play all day long? That would hurt my back and i've been playing for 18 years. Depends on the weight of the guitar though really, my ESP is quite heavy so after a 3 hour session in the studio my shoulder can feel a bit tired.
Never. You're having back pains because something's causing it, I can't believe I have to tell you that it doesn't 'toughen up'. Your fingertips toughen because they callus. Your back won't callus or strengthen from playing guitar.

See a doctor or a physiotherapist if you're concerned.
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You have several options.

Get a weight supporting strap. You can get some that distribute the weight better than a standard strap. This can be as simple as a really wide strap or a harness strap.

Get a lighter guitar.

Do some back exercises to strengthen the muscles in your back.

Play your guitar higher up your chest.

Play sat down.
I have been playing for a good few hours standing up. I was just under the impression that my back would somewhat get used to it, thanks for the answers though
"I think the most important thing about music is the sense of escape." - Thom Yorke
I can play all day and not notice the weight.

You might just be standing with crap posture, sand up straighter, put your strap to te right length.

Dont be hunched over looking at the fretboard.

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Stretching helps
Posture, like Tom said bad posture, whether your playing guitar or just standing can have a drastic effect on your back.
You CAN do exercises that strengthen your back, core, shoulders and legs, which should help you out overall.
I have personally never had any back pain from playing, but there are things that can be done to help alleviate unwanted stress on your back.
You might want to also determine if it was from playing or if you have ever had pain before and just dismissed it.
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Do some core exercises, get a weight distributing strap, and have good posture. Many people know about keeping their shoulders back, back straight, etc. Fewer people realize that how you stand on your feet impacts your posture tremendously. Start with your feet a few inches apart with both your feet pointing forward. Rotate both your feet 30 degrees laterally. Next, take a slight step in the direction you foot is pointing with your dominate foot (opposite of dominate hand). Balance your weight on this and you will have less back fatigue. I learned this years ago from my violin teacher, now I rarely have a sore back.
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If your back is hurting (or any part of your body, for that matter), stop what you're doing; something needs to change. When you're feeling pain or overly tired, that's your body telling you that hey, you're doing something that is wrong and bad for it. Finger callouses are a freak exception because that's more just skin rather than your system as a whole.

Take it from someone who's ****ed their back up with too-heavy guitars, weak straps and bad posture. I thought I was just being a bit of a weakling, then one day nope, spine's given out, eight major nerves are trapped, I can't walk for a year and have to have a series of very painful surgeries to have parts of my spine removed just so I can walk and get some feeling in my left arm again. The idea of me playing lead is now a complete joke; I can play your basic powerchord rhythm fine, so long as I use a light guitar and don't jump around.

Shit ain't fun.

There is nothing in the world—absolutely nothing—that is worth compromising your health. If something hurts or tires you, stop doing it immediately. Adjust, work out what you're doing wrong and fix it. If that means buying a bunch of different straps to try, using a shorter or longer strap to put the guitar in a different position, buying a lighter style of guitar or doing less stage acrobatics, so be it.
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Yea, stand up straight and sort out your core.
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Sure upgrading your gear will make you sound better.

Just remember that the body needs to always remain in balance. You cannot strengthen your back without strengthening your stomach.
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You need to take a break every hour or so. This is one reason bands often break up long shows with solos, set changes, chants, etc. It gives people a chance to rest, stretch, etc.

And try different straps to find one that works well with your shoulders.
Upper, mid, or lower back pain?

If it's from poor posture, you should change that. If you just have a weak back, then you should look into exercises to strenghten it.

The pain might go away over time, but I wouldn't count on that.