Unknown, but we have some lower bounds.
From the latest series (emphasis mine):
DOCTOR: Proposition. Davros is an insane, paranoid genius who has survived among several billion trigger-happy mini-tanks for centuries. Conclusion? I'm definitely having his chair.
-- Series 9 Episode 2: The Witch's Familiar
That means Davros at least has lived for centuries. As for the Daleks, we have a lower bound of well over 50 years from an episode in the first series of New Who (emphasis mine):
DOCTOR: A nightmare. It's a mutation. The Dalek race was genetically engineered. Every single emotion was removed except hate.
VAN STATTEN: Genetically engineered. By whom?
DOCTOR: By a genius, Van Statten. By a man who was king of his own little world. You'd like him.
GODDARD: It's been on Earth for over fifty years. Sold at a private auction, moving from one collection to another. Why would it be a threat now?
-- Series 1 Episode 6, Dalek
Other than these lower bounds, we don't really know. The question was posed on another site and got the following answer:
It is currently unknown but quite possibly forever.