If you are looking for Just Udaipur 2 days would be enough but if you are looking to explore the culture and nearby places it would require a week. You can enjoy Kumbhalgarh, Ranakpur, Chittorgarh and Jawai if you extend and explore nearby places.
2 days for siteseeing and 1 day for kumbhalgarh fort. one can also keep another to go in at a relaxed pace. so min 3 and max 4 days are required to complete all of udaipur
2 days enough for udaipur Nearby places not to be missed are Kumbhalgar and Ranakpur main temple. Haldighati is also nearby. You can also visit Nathdwara temple.
If u have more time then Chittorgarh is also an option