What X Equistris said. Of course, they already had their tents, so a town would take a bit longer. And the building materials would be a variable. And if the people involved were soldiers with engineers that's one thing, but if they were ordinary civilians, that's another. But I figure a domicile every two days, presuming skills and tools and resources, is not unreasonable. If we're talking families, then five to a house, so that's 20 houses, so forty days. You could probably trim that or extend that without offending your reader's sensibilities.
In Egypt, they had villages built just to house the workers needed to build a complex of buildings or tombs. When the work was finished, the town was no longer needed and they abandoned it. These towns weren't that much smaller than 'actual' towns. And this was in the bronze age, they didn't have iron yet (I'm pretty sure that's the case).
The point I want to make is this: if you have a powerful central government and it wants something built, then it's fine to think up heavy handed maneuvres.
While I'm not disagreeing with skip.knox and 20 home in 40 days as an average rate, I think it might be more like 0 homes for 20 days and then a lot of buildings going up almost at once.
The site would need to be explored/surveyed... Ground to be cleared and levelled... Foundations made/laid... Material/wood collected and got ready... and all the things I haven't thought about... Sanitation? Clean water?
What I am imagining is something like a Saxon Pit house... for something closer to a Roman Villa - a whole lot more work would be needed...
If you are thinking about soldiers and the like then larger and fewer barracks style housing could be built. You might need only 2-5 larger buildings, maybe just one...
And I might be tempted to say that large building projects would be easier for 100 people to work on efficiently...
At any one time there would be a finite number [10?] that could physically work on a smaller building house/hut/home for 5 people; but a larger building for 25+ people and perhaps most could work on that at the same time...
Don't forget about roads, you would have to have at least rudimentary roads for travelling merchants. A town that size could not be entirely self sustaining unless there where both mines, plantations, farms etc. in the vicinity as well as a lot of knowledge in the population.
Pots, anvils, hammers, farming tools, eventually has to be replaced, as does clothes. Even if you have a skilled blacksmith you would still need to have iron to work with. There would need to be a way to trade. You could solve that problem though by placing it near a wide river.
There are different techniques you use, not only depending on how prepared you are and how many hazards you have to prepare for, but also how long you need the place to last. Overall planning, deeper foundations, heavier materials... there's a difference between say settlers trying to start farmhouses for a new life and what a miner or trader would throw together to stay warm for a few months of work.
Also, I don't think an empire is bothered with something as small scale as the building of a village. The splitting up of population is something that occurs naturally with growth. The only real involvement that's required from the government is the establishment of tax collection by the one in charge of the region.
And since this type of settling is something that people see coming long in advance (the amigrating families need to be decided on etc.), they will have time to prepare a good stock of tools. They might even take a smith's apprentice (I know I would)., and maybe even some other apprentices. They'll have the site already picked and mapped out and they'll go there ready to work their asses off.
That's how I see the establishment of a new town. It should be in a hospitable season (or the most hospitable, if the region is inhospitable), and construction should be done by the entire community, in a burst of activity that lasts a few weeks I'd say, with all the manpower concentrated on a few projects at a time. Think of the barn raisings that are still done in some places in the world (or that used to be done). People would come from all over the town to help pull up the walls.
Apart from that, nobody ever said that the town needs to be built in one go. I think it's more likely that they sent some pioneers ahead to chop wood etc.
For an idea of what *could* be done, read about what Caesar did at Alesia. Granted that was unusual to the point of being epic, but it gives an idea of a sort of upper limit.
To argue out the other side of my mouth, the points others have made here are germane. Ancient societies generally did not build towns. Towns evolved. The closest I can think of for building from scratch would be those Greek colonies. Unfortunately, we have little more than place names for details, so you won't get any guidelines there. But building a colony, whether for population overflow or for discharged veterans, is one of the few cases I can think of in which a town would be built from scratch.
there is a story in Japanese history of a general prefabbing a fortress (wood ofc) and having it built in one night, thus gaining control over a crucial plain