Can dementia come on suddenly?

Here I am again - requesting information & reassurance. Three days ago my 93-year-old father became very confused & agitated. The ER visit showed only dehydration & gave him IV fluids & discharged him. We took him back to the Independent Living section where he resides. He continued to be confused & restless. Apparently no UTI evidence from the routine urinalysis done, but the staff at the facility felt it couldn't be anything else. After the RN spoke to his MD, he was started on Macrodantin. He has shown a little improvement, but is still forgetful & agitated. And he knows something's wrong & keeps asking if he's going to get better. I guess I'd like to know if dementia could happen this abruptly or is it more likely a UTI? He cannot be by himself at this time & is temporarily in the Memory Care center at the facility in the daytime & I have an aide with him at night. It's very disturbing since he was VERY sharp prior to this - meticulous with his hygiene, able to talk appropriately about current events, etc.
If this is a UTI, what is typical recovery time? The condition has made him twice as anxious & afraid as is usual for him & has caused him to hang onto me as his lifeline. I can't be over there every day & I'm getting stressed myself. It's like a switch was flipped & he's a completely different person! Feeling guilty. Thoughts please!