Considering they have a lot of SFIV stuff to rehash, I'm pretty sure they can release 8 characters per year plus additional stuff like costumes, colors, stages, music themes, new game modes, rebalances, etc.
It doesn't mean they have to develop 8 characters per year, because in most games a big portion of the DLC if not all is considered as part of the whole project so shares the same budget and development time, which means that most of the work for these DLC characters will be already done or considered before release (at least the first season).
I'd make 1 year seasons, that would be:
-Months 1 and 2: break between seasons
-Months 3-6: 1 character per month (so 4 extra characters)
-Month 7: small gameplay rebalance tweaks, new costume colors, new stages, new music themes
-Months 8-11: 1 character per month (so 4 extra characters)
-Month 12: big gameplay rebalance update, maybe with new game modes and new costumes
The proportion of these 8 extra chacters for each season would be: 4 returning USFIV chars, 3 returning SFA3 Max/3rd Strike/etc chars, 1 'brand new' (first time playable in SF, Haggar, Zeku or Goutetsu or chars from other FG would fit here) chars.
Each season may have a special theme, like 'Linking with SF3' (2016-2017), 'SF 30th anniversary' (2017-2018), 'Final Fight' (2018-2019), etc.