When you initiate a Power Down it notes your current Steem Power and sets a fixed 1/104th part in in Vests that gets sent to you every week (you can see that recorded on steemd.com under Vesting withdraw rate).

That number remains fixed untill you do a Cancel Power Down and Reinitiate it again in order to get an up to date value of Steem Power (VESTS) you have available and do the calculation again.

So if you just Power Down and don't cancel and repower again every week you will still have some Steem Power left even if you wait the 104 weeks to pass, because of the interest rate you have and probably the interaction you do in that period of time ot steemit.