It depends on what kind of stuff you want to see.
You can easily spend five days in and around MAA.
City sights = one day or two days
Mahabalipuram plus lunch / dinner at Taj Fisherman's cove = one day
Kanchipuram (temples, silk shops) = one or 1/2 day
Beaches = evenings
Snake park, crocodile park, bird sanctuary, zoo (all different places)
Catch a Tamil movie.
Pondicherry = one day
EAT! (Every day
)
Plenty to do.