This is a hard question to answer. The longer you spend on safari, the more animals you'll see, and the greater the variety you're likely to spot. If you're hoping to spot the Big Five, for instance, one day probably won't cut it. But if you give yourself four to five days, you might well manage it.
What we can say is that you need to factor in travelling times when heading on safari. For instance, from Kilimanjaro International Airport (gateway to the Northern Circuit), you can reach reserves like Arusha National Park and Lake Manyara National Park within hours and go on safari that same day. But Serengeti National Park, on the other hand, is quite a drive, so you should factor in a day of travel each way. Fortunately, the drive to Serengeti takes you past Ngorongoro Crater, which is a MUST, as well as some other attractions really worth a visit like Olduvai Gorge.
Please chat to us for advice on how long is needed based on what you want to see and where you want to go.