What sativas are like that where you can flower longer than 2-3 months?
Well, true tropical sativas can normally take 16 weeks to flower, and some even longer.
Remember, these plants grow near the equator where you never get less than 10 hours of light per day, and the temperature rarely gets below 70F.
But these plants are ill-suited for indoor growing, and very few people try.
Many of these plants will have different harvest "windows" where most of the buds mature (say at 12 weeks), then the remaining buds undergo a growth spurt, and aren't ready for three more weeks, etc.
And there are lots of plants that will mature, but will keep growing after maturity and never really "finish" flowering. Just to name two popular ones, I've seen reports on both AK-47 and Northern lights where growers just let them go to see what happens, and they just kept growing and flowering way past 12 weeks.