Don't know, but if history serves as any guide, until December. They usually shut down both pipes in late fall for maintenance, then use one pipe at a time until spring, performing yet more maintenance on each unused pipe one at a time. When they run one pipe it takes about 700cfs, when they run two it takes about 1200. Why two don't take 1400 I don't know.
As you may know the Shoshone Power Plant has senior water rights to call for 1200cfs in Shoshone. However, because of the above maintenance schedule they rarely invoke that right, and almost never in the winter months.
Hiked Barrel Springs rapid today, went great in a playboat. Its pretty mellow right now, much more so than even 400cfs ago at 1200cfs.