What are the fastest and slowest gestation rates possible in a humanoid species and/or a human sub-species? What does the gestation rate say about the species and vice versa?
Gestation period lengths mostly relate to size and how developed newborns need to be to survive. This is somewhat related to the species level in the food chain. Quick gestation with small relative body size is typically only found in lower level food chain animals. The mouse is the first to come to mind. They are clearly a "feeder" species. Very long gestation periods seem to belong only to the largest of species, which consequently have few predators. The elephant comes to mind, needing longer than two years to produce their 200 lbs newborns. Whales also need significantly longer than other species.
Humans average about 9 months, as you know, however, that is only one in a long list of traits that contribute to their domination in the food chain. At least, not in the recent evolutionary past. Comparing to other top predators, humans are relatively long. Comparing to relative sizes, human gestation is relatively long there as well. Some reasons scientists give is slow relative metabolism and a far more complex brain. Compared to chimps, the closest evolutionary relative to humans, chimp gestation is about the same time, however, chimp newborns are far more developed than humans.
If this is to be explained by evolutionary pressures, rather than the chance of genetic drift, then it's easily believable that a humanoid species has a gestation as little as half as humans. Conversely, it's just as easily believable that a humanoid gestation is twice as long, but newborns are far more developed.
Another factor is how much effort parents put into their offspring's survival. Humans put a great deal of effort in, resulting in a post-natal developmental period of nearly two decades. Other species are far, far shorter than that, but generally the longer they spend on their offspring, the more post-natal development there is rather than in utero.
So you also need to determine how developmentally advanced these humanoids are. If they are highly developed at adulthood, it is likely they have a long gestation and a very long "childhood". However, if they are primitive, they will need less.
The human average gestation period is 270 days.
A list of mammalian gestation rates can be found here.
Some of our closest relatives, gorillas and chimpanzees have similar if slightly shorten gestation periods; gorillas - 257 days, chimpanzees - 240 days.
Judging by the list the gestation period is roughly correlated with size of the animal, larger animals have longer gestation periods. So a smaller sized humanoid would have on average a shorter gestation period.
There are of course other options:
The marsupial route: emerge from the womb into a pouch, this allows much less development before "birth". The largest kangaroos have gestation periods of only 33 days, but the neonate will stay in the pouch for another 235 days before emerging. So total time is still 268 days, comparable with other similar sized mammals.