I am not sure when although I would imagine it is a continual, gradual thing.
My guess is this is to offset the inflation cost to all steem holders meaning that it would probably be tied to the rate of expansion on a daily basis.
But that is just a guess.
I don't think it's changed at all since I've been here (about 3 months now) so I'm not really sure what the deal is with it. Maybe it's possible that it doesn't work that way anymore maybe after the inflation rate was lowered? Either way I'd really like to understand it better.
Yesterday was 487.244...now it is 487.26.
So it did move up....I will have to watch and see how much it moves over the weekend.