How long do you have to wear night guard for bruxism?

Months to year: It depends largely on the degree of the problem, the body's response to the treatment, long enough treatment that recurrence not likely. Must be guided by the professional treating you for this problem.

All night: Bruxism, or tooth grinding, usually doesn't go away, though it's frequency and intensity can vary depending on your daily stress level changes. If you wear one, try to keep it in place all night if you can tolerate it. Unless your dentist tells you otherwise, plan on wearing it for the forseeable future...It can't hurt, and it's protecting your teeth from possible cracking and wear.

Rest of life: A night guard only runs interference between your teeth to prevent damage. People who brux their teeth will always brux just at different rates depending on how much stress they have in life.