D.W.
I'm really surprised that your doctor hasn't had you come into the office to collect a urine sample - it's impossible to determine if it's a UTI vs. bladder infection without the culture, and the antibiotics approved to treat each are slightly different.
Different antibiotics take different amounts of time to build-up to therapeutic strength in your body depending on several factors such as how frequently you take it, the strength, etc. Ask your pharmacist about some of the products recommended.
People will tell you cranberry juice, but it's only effective in preventing UTIs and has to be 100% juice which is really hard to find.
Good luck! I hope you get relief soon.