For me the large size of toothpaste tube lasts me up to 2 months and I brush my teeth twice per day.
When brushing your teeth you only need a little bit of toothpaste and not a whole lot of toothpaste so the large size tube of toothpaste should last from 1 to 2 months depending on how often you brush your teeth and how much toothpaste you use.
I've heard of some people say they have one tube of toothpaste last a year or two years but they must not be brushing their teeth that often.
As for how long does the tube of toothpaste last before it expires and goes bad the tube of toothpaste does expire and eventually go bad so it's best to use the toothpaste up before the expiration date.
However even when the toothpaste has expired the toothpaste is still safe to use but the expired or old toothpaste may not be as effective at cleaning your teeth properly like fresh toothpaste would be.
Toothpaste lasts from 12 to 18 months past the toothpaste expiration date so it's best to use the toothpaste up before that time has passed.
But you can still safely use old expired toothpaste even well past those 18 months without getting sick.
Old toothpaste is not harmful to your health.
But it's best to use fresh toothpaste to get your teeth really clean.