How long does it take to gives again into circulation after loss Bitcoins ?
This question is based on a false assumption: Lost bitcoins are not returned into circulation.
The problem is that one cannot distinguish lost bitcoins from saved/stored bitcoins, as it is impossible to prove that nobody has the private key to a given address.
Eventually once all of the bitcoins have been mined the design calls for a permanently-finite system and eventual deflation
Also, Bitcoin users are faced with a danger that doesn't threaten users of any other currency: if a Bitcoin user loses his wallet, his money is gone forever, unless he finds it again. And not just to him; it's gone completely out of circulation, rendered utterly inaccessible to anyone. As people will lose their wallets, the total number of Bitcoins will slowly decrease...
...That being said, there is a mechanism in place to combat the obvious consequences. Extreme deflation would render most currencies highly impractical: if a single Canadian dollar could suddenly buy the holder a car, how would one go about buying bread or candy? Even pennies would fetch more than a person could carry. Bitcoin, however, offers a simple and stylish solution: infinite divisibility. Bitcoins can be divided up and trade into as small of pieces as one wants, so no matter how valuable Bitcoins become, one can trade them in practical quantities.