Taken from steemit.com FAQ - Powering down - If you have STEEM Power, you can power down to turn it into liquid STEEM over a period of time. The system will transfer 1/13 of your STEEM Power to STEEM each week for about three months (13 weeks), starting 1 week from the time it is started. However, you will lose your influence in the network proportionally to how much is powered down, so think about it carefully. Power downs can be stopped at any time.