On the hardware link the schematic dates for the Duet v0.8.5 and v0.6 are:
V0.6 18/10/13
V0.8.5 07/07/15
On the hardware overview it mentions the release date as a August, a month after the schematic files were dated. Probably similar for the other releases.
Edit: I was wondering about release schedules myself this morning!
v0.6 lasted around 21 months
v0.8.5 lasted around 12 months
v1.. has lasted about 17 months
Some of the changes being muted for the next major release are massive changes, such as CAN-BUS. Can't see that being released imminently. Having said that I'm yet to fully explore the capabilities/limits of my v0.6 board!