Is there any expiry date to claim our rewards? What's that 3 days withrawal time mentioned in the "Wallet" section of new comers on steemit when they try to click "Transfer to savings" option??

Secondly , is it so that it is necessary to claim our rewards within 7 days of posting our any post? What's that 7 days and 3 days policy on this Steemit platform for new comers???? :)

Thank you.
Hoping to get my all answers in the next new post from someone...!!!