Once you start a power down it will take 13 weeks to fully convert your steem power to steem. Every week 1/13th of the amount you want to power down will be converted to Steem.

This is to make sure that people don't cash out SP instantaneously and hence have long term interest of the platform on their mind.

This is also a security measure as you can initiate an account recovery in case you realise your account is hacked. The hacker cannot get away with all the SP as it will take time to be converted into Steem which can be transferred to exchanges.