A Trust can last almost as long as its creator wants it to last, but there are some limitations and those vary from state to state. In most states, a Trust cannot last longer than about 21 years after everyone alive when it was created has died. Some states, however, have done away with this rule in favor of longer running trusts or more simplied calculation. In California, a trust may be permitted to continue beyond the 21 year limitation as long as it ends within 99 years. Delaware allows trusts to continue up to 300. Some states have no expiration at all.
A Trust may also end early if it runs out of property, or if the value of the property is so low that it doesn’t make sense to continue as a Trust anymore. It can also end if a Probate Court orders it to end, but that usually only happens in special circumstances.