Agra is roughly 200 to 230 km (125 to 145 miles) from Delhi, a drive of about three to four hours on the Yamuna Expressway. With an early start it is entirely possible to see the Taj Mahal and Agra Fort in a single day and be back in Delhi for dinner.
The day works because it is planned, not rushed. You travel in your own luxury air-conditioned car with a driver who knows the road, meet a licensed guide in Agra, and visit in an order that avoids the worst of the queues and the midday heat.
If you would rather not rush, the same route works well as an overnight stay in Agra. Ask us and we will adjust it.