How often does this game need expansions?
In a traditional CCG games, 4 months feels about right because players almost explore the full range of killer decks in about that time range and it starts to just get a touch repetitive.
KeyForge with the "Unique Deck Game" may actually have players wanting to spend more time between expansions. The sealed play is certainly going to be used, bring and play and some variations on that, even just casual games at the FLGS will have a degree of newness to them. I expect players will at first chase decks they want, they will play them in preference to their other decks but over time will experiment and enjoy taking the deck of their second or third choice. I also wonder what options will come through the Organised Play Team with respect to Leagues and Challenges for groups to expand of the game variety more.
I hope that the expansions will be careful to minimise the power creep in each set, so that way if I want to I can return to those decks from previous sets that I enjoy to play.
I know I am strange, but I hope to get a few rubbish decks and play them to death. Perhaps there is just a part of me that wants to believe I can learn to play a poor deck well and see for myself that pilot skill can overcome luck, but also because I think you learn more for losing. I find that in the three games I have played it is fun to play these crazy cards that see you grab a whole mess of amber, but it is also just as much fun to see how your opponent runs his crazy cards together to rip that amber away from you. After all, at the end of the day, your opponents deck was a random assortment of cards too.
At this time I think a 6-8 month cycle would feel like about right. If Asmodee Digital gets the App running online play, I would lower those numbers because players will find they can get a much higher game count than they could with analogue game play alone.