I like the idea of 1 month full strength (so that you dont have to adjust more than once a month to maintain full strength, followed by a half life period. I would say 1 year half life.
If one of our main goals with this method is to remove stale/non-performing delegates, I don't think we will want anything longer than a 1 month half life.
A 1m BTS vote will still have ~ 250k strength @ 3 months - a lot can happen (or not happen) within that time.
I think the countdown reminder with the free monthly vote would be enough to encourage the voter refresh and engagement we are looking for without stretching it out too long.
It would also be useful if the witness was able to view how many / what percentage of their votes are in decay to allow them the ability to communicate with their supporters via forum/email/whatever .
^^^^This will allow a similar function to that of our transaction referral system does - in outsourcing the incentives more greatly toward the witnesses to encourage continuous, active voter participation.