You can instantly power up STEEM Power here on Steemit. However, the process of powering down has a certain mechanic behind it and the involved timeframes might be somewhat longer than expected.
Are you planning to convert your STEEM Power to actual STEEM and transfer it to an exchange for trading activities? Well, you might be in for a surprise ... ! First, you start the process of powering down by going to your Wallet, click the little arrow behind the amount of STEEM that represents your STEEM Power, and then click on 'Power Down' on the little pop-up menu.
Subsequently, you'll see a message that the next Power Down will occur in 7 days. This means that it'll take 7 days for the Power Down to actually occur. Now, I can hear you ask: "Seriously? It takes a whole week to transfer my STEEM Power into actual STEEM??"
No, that is not the case.
It takes 13 weeks to transfer the whole amount of STEEM Power into 'real' STEEM. Each Power Down 'event', which occurs each week, transform 1/13th of your initial amount of STEEM Power into STEEM. If one has 130 STEEM Power available and initiates the powering down, each week will see a transfer of 10 STEEM Power into STEEM.
I found out about this as I tested this mechanic in the last week. Although it might have been more convenient if I simply would have seeked the information available in the FAQ. Personally, I think this system has pros and cons; the most prominent con is that you cannot swiftly allocate much of your funds to another location (exchange?) for trading activities or cashing out. The most prominent plus is it prevents the Steemit 'ecosystem' from a disastrous collapse; a quick declining value of STEEM could cause panic and make participants here on this site withdraw their funds quickly. The '13-weeks system' prevents such developments.