Although there is no exact answer to this question, because there are so many variables to consider, I will do my best to explain the process involved and the timeline to be expected.
The size of the pool, the amount of masonry, the material the pool is excavated out of, and many other factors play a roll in the construction process. However an average pool or pool and spa combination in the central Texas area to include Austin, Round Rock, Cedar Park, and other areas nearby should take between 4 to 6 weeks. This would include a small water feature, decks, and possibly an outdoor kitchen. However let us assume you have the exact same pool, but with a large rock grotto and slide, including an all tile interior, and a large pergola on it. In this scenario you would add about a week for the grotto, another week for the pergola, and an additional week or two for the tile interior. Other issues to remember are weather, back order of certain kinds of specialty materials, and things like special engineering and permitting. For every rain day you must allow another two days to allow the ground to dry out otherwise you can do more damage than good trying to maneuver around the project. If you want specific items like tile, or certain equipment, there are times that they can be on back order, so try and be flexible with those selections, and finally if your project requires a level of structural heroics like peering, extensive retaining walls, or other things, be aware that these will take additional time as well.
Permitting is another issue to consider. If you live in the city of Austin, be prepared for a minimum of 2 weeks for permit approval. If you have impervious coverage issues or if you have large project be prepared for a longer permit approval time. If you live in Lakeway the wait can be 6 weeks or longer. I have addressed impervious coverage and other unique Austin issues at my personal Reliant Pools blog.
Finally be aware that the project isn’t necessarily complete with the pool, there might be fencing, irrigation, landscaping, landscape lighting, and additional things you want to have done over and above the pool itself that can take additional time as well.
Regardless of what size project you are looking to build, your contractor should be able to explain any and all of this to you, and give you an approximate time frame in which it will take to complete.