Almost indefinitely, if properly stored.

Thank you very much for your answer. Both cell lines are more than 1 year in liquid nitrogen.  I hope that everythink will be OK with cells. Thanks

No problem, we routinely use cell lines that are way more than 1 year old. In the first week(s) after thawing if you notice a slow growth rate it could be useful to culture them in 20% FBS.

you can put your cells in liquid nitrogen for ever, but when you want thawing them try to keep them away from stress