How long would it take for a 15 week old chihuahua to adjust to a new home and environment?

It depends on the environment and the chihuahua. Try to start setting up a training pattern and also make the environment as calm and comfortable as possible for them. Confortability is a personal thing, there is no standard. Sorry if this doesn't help.
Good luck!
Heather

It could take a while. Dogs accept new things much more readily before 12 weeks. At 15 weeks, it could be starting into one of its fear phases. Be careful not to overwhelm. However, dogs don't read the behaviorists books, and often fail to do what the books say they should.