Well, they have played loose with the timeline of events before. The fourth movie obviously intended TD to happen during it (or shortly thereafter) which is why Han split after the prologue sequence- but they decided to insert two other movies inbetween them afterwards. And like was pointed out- the epilogue with Vin in TD is detached enough from the rest of the movie that there could plausibly be several movies set after the Yakuza stuff but before Dom shows up if they really wanted to (though I think the only limiting factor would be a consideration towards Lucas Black's age: it'd look mighty strange for him to suddenly age 10+ years between his "first" appearance and his "next" if they are in close proximity, chronologically- not to mention Dom would simultaneously be 10 years younger! ). Because of the latter though, and the relevant plotline with Han, I think it's safe to say it'll only be one movie inbetween at most.