5 days worked well for us.
1 day to Matagami, 1 day up the JBR stay in Radisson, 1 day to Chiassabi/James Bay and the Hydro tour, stay in Radisson, 1 day back down the JBR to Matagami, 1 day to GTA.
Gives you lots of time on the JBR for photos, rests etc... if you're so inclined.