This is a very good question. When we stand for too long in one place the weight bearing surfaces (i.e. joints and discs) are taking the majority of the load. Over time this can cause stress to the joints and discs as well as reduced circulation to your lower back resulting in low back pain. If you have any degenerative changes (i.e. wear and tear) to your joints or discs then the amount of time that you can stand in one place may also be limited. Tightness in your hamstring/hip flexor muscles or increased tension in the muscles/fascia in the lower back can also increase the compression on your lower back. Low back pain when standing can also be caused by weakness of the core stabilizing muscles in the spine. A thorough examination of your lower back needs to be conducted by your a Physiotherapist to see what solutions are best for you.
In the meantime, if you have to stand for a longer period of time you should try to place one foot on an 8 – 10 inch rise. This puts your lower back in a better position to accept the weight for a sustained period. Also, squatting down and/or performing some pelvic tilts will also stretch out the muscles and increase circulation to your lower back. Wearing comfortable shoes (no high heels) with a good arch support is also important. Bottom line; we are not designed to stay in one position for an extended length of time. Our body needs movement to bring fresh circulation and nutrition to the tissues in our body in order to prevent pain.
I hope this helps you. Please feel free to send me an email if you have any further questions.