I've been on antibiotics for a severe sinus infection for a week already, and its not any better. how long can this take?

Maybe need to switch: If you have been appropriately taking the antibiotic (not missing doses) and doing the other recommendations like saline irrigations, nasal steroid spray etc and you are not better at all, i would talk to your doctor about switching them as you may have a resistent bug. This is especially true if you have been on another antibiotic in the recent past, as resistance could have developed.