Mahe is not a beach with an island attached. It is a granite mountain range roughly 27 kilometres long with beaches around the rim, and that geography decides both how long you need and, more usefully, what order to do things in. Most visitors give it two nights on the way to Praslin, photograph Beau Vallon, and leave believing they have seen it.
Four days is the number that works. Three gets you the north and the middle and forces you to skip the south, which is the half most worth the drive. Five is only justified if you are diving seriously or walking above the ridge line. Beyond that you should be on a ferry, because the premise of the Seychelles is that there is more than one island.
There is a second reason to plan in days rather than in sights. The archipelago sits between two monsoons, and they take turns sheltering one side of Mahe and working the other over. A route copied from somebody who visited in the opposite season will send you to a beautiful beach on precisely the day it is unswimmable. So what follows is not a fixed sequence. It is four days you shuffle according to the wind you arrive in.
Work out which monsoon you are in before you plan a single day
From roughly May to September the southeast trades blow. The north and northwest coasts come into shelter, which means Beau Vallon, Port Launay, Baie Ternay and Anse Major are the calm water. The south and southwest take the weather head on. Anse Intendance, the beach that appears on most of the postcards, develops real surf and rip currents in these months, and it has no lifeguard.
From roughly October to April the northwest monsoon reverses the picture. The east and south coasts settle down, so Anse Royale, Anse Takamaka and Anse Forbans become the swimming, while Beau Vallon is the one taking wind and chop.
Between the two systems sit the transition months, April into May and October into November, which is the window most Seychelles operators steer visitors toward. The water is at its flattest, snorkelling visibility is at its best, and the drifting seaweed that the trades push ashore largely stops arriving. The trade-off is heat, stillness and the year's highest rates.
The scheduling rule that falls out of this is one sentence long: swim on the sheltered coast, sightsee on the exposed one. Follow it and every day below works in any month. Ignore it and you will spend an afternoon standing on sand you cannot get into.
One more piece of arithmetic before the days themselves. Your arrival does not count as a day. Long-haul flights into the airport at Pointe Larue, just south of the capital, tend to land late in the evening or before dawn, and the first morning is generally spent asleep or waiting for a room. Build the four days after that.