The most useful planning question is not where to go but how long you have. In Kenya, each extra pair of days changes the trip in a fairly predictable way.
Two days: one destination, two half-days
You reach the Maasai Mara, get an afternoon drive and a morning drive, and come back. It works, but most of both travel days go on the road. Take it if it is all you have; take three days if you can.
Three days: one full safari day
This is the shortest format with a proper day in it — out after breakfast with a picnic lunch, back at sunset, nothing to drive to. That single day is worth more than any third destination you could add instead.
Five days: the Mara and Rift Valley lakes
Five days lets you add Lake Nakuru for rhino and Lake Naivasha for a change of pace. Naivasha is the part people underestimate: after the Mara and Nakuru, a boat ride and a walk in Hell's Gate change the rhythm. This route leaves out Amboseli.
Six days: the Mara, Nakuru and Amboseli
The six-day route uses its extra day to reach Amboseli rather than stop at Naivasha. It gives full safari days to both the Maasai Mara and Amboseli, with Nakuru for rhino between them. Choose it when elephants beneath Kilimanjaro matter more to you than the Naivasha and Hell's Gate pause.
Seven days: add Naivasha to the broader circuit
Seven days combines the Mara, Nakuru and Amboseli with the Naivasha and Hell's Gate component. It is the broadest of these three first-time routes, with four different landscapes across the week and a long southbound transfer before the full day in Amboseli.
Nine days or more: go north, or cross the border
Beyond a week, the interesting choices open up. Samburu and Ol Pejeta in the north give you an entirely different species list, or you can cross at Namanga and pick up the Serengeti and the Ngorongoro Crater.
Count travel days honestly
A quoted length includes the days you spend getting there and back. A five-day safari is really three and a half days of safari with a travel day at each end, and that is normal — but it is worth doing the arithmetic before you decide five days is enough.
Long-haul arrival days are worth treating as written off. If your flight lands in the evening, plan a night in Nairobi rather than starting a six-hour drive on no sleep.
One rule that holds at every length
If you have a spare day, spend it on a second night somewhere you are already going rather than a third destination. Time in one place beats distance covered, every time.