You can withdraw your steem power at any time by powering down. It will be converted to steem tokens meaning the value of your steem power is whatever the value of steem is on the market at every interval you power down. The whole power down process if you choose to initiate it is 13 weeks. That means if you have 13 steem power every week it will be converted to 1 steem until the end of t. If you choose, you can stop the power down at any time.

You can learn more about it here

https://steemit.com/steemit/@cryptos/how-to-power-down-your-steem-power-the-smart-way