Seconding the different antibiotic idea. The best procedure is to treat with one of the more common antibiotics and also run a culture to see if the bacteria respond to that antibiotic. Sadly what often happens is that a common antibiotic is prescribed but no culture is run until you go back and say "hey, this isn't working".
If the antibiotic is effective against the specific bacteria, you should start to see an improvement within 7 days (when it comes to symptoms such as pain, and increased need to urinate) although any infection can cause dementia to take a down turn and it could be weeks before the person comes back to where they were before, even if the infection is out of their system.