Basicly it would be just a slightly peppier yak-9. Probably something on the order of the 190A-8 (Yak-9) compared to the A5 (Yak-3).
Infact, thats actually a pretty good comparison, since the A5 is faster at low alt as well.
And Karnak, they announced the B-29 in mid-late January(?), and it was released sometime around june. Thats around 5-6 months.
Granted the B-29 probably had less things infront of it on the list, but they also made it sound as if they were starting almost from scratch ("we are starting work on the B-29"). It was also larger, as you said. But with the Me-410, they just asked us what we wanted next. This could mean they had no work on it done, a decent start, or nothing at all. However its smaller, and closer to the Bf-110. However (if they want to do it right), they'll have to also develop the new Mk 103 30mm.
Probably about the same time, I would guess.