With due respect to some of the above posters, 12-12 is irrelevant.
Most strains grown outdoors will actually start flowering under only about 9-10 hours of darkness and will be more than 2/3 of the way through flowering at the time of the 12-12 September 21st equinox. In fact, plenty of "early" strains will be *DONE* by the time the light gets to 12-12 outdoors!
If your plants only started flowering in late September you'd have a bad problem in most of the country, where frost can come as early as November!
Presuming the question is "when is the latest I can plant outside and still have the plants mature", the answer depends both on where you are growing (ie when first frost comes), and what strain you want to plant.
Speaking in general, most strains finish up growing outside by mid-October. Allowing 3 weeks for sexual maturation of the plant from seed and another full 9 weeks for flowering maturity, you can plant outdoors FROM SEED into mid-July in most cases and still harvest perfectly mature (though not all that tall) plants at their normal time in October.
Obviously, if you're planting an already mature clone or plant grown indoor from seed you can get away with planting it outdoors later.
Also, many strains take less time to mature than 9 weeks, and/or start flowering earlier. So again, as long as you pick a strain that doesn't take extra-long flowering, you can get away with planting later, even into August.
Lastly, if you're lucky enough to be in a nice sunny state like CA where frost comes late (or not at all), you can probably get away with planting even LATER yet , even into September if you had to.
Again, the main problem with planting really late in the season is just that the plants won't get very big, and yields will be low. But as long as there is no frost, the plants will still flower and mature, even under 10 hours (or less) of daily light.
Bottom line, its not even CLOSE to being "too late" to plant now in May! There is still PLENTY of time to put seeds (or clones) in the ground and grow nice big plants if you wanted to do that.