How long until Platinum?
Although it wasn't nearly as popular the second time around, Twitch Plays Pokemon moved on from Red and switched to Crystal at the start of this month -- only to complete it late last week. We're looking at roughly 20 days per game completion based on the sample size so far.
The group is now working their way through Emerald, and they are now including a "current party" bar at the bottom to show the current group composition. Progress!