India is a large country. If you set up a tour to see all of the sites of India, it would take a long time and a lot of planning would be required. I imagine like many other countries, it would take a few weeks to visit each tourist attraction and the main places in India as long as you didn’t linger in a certain place for very long.
A month would be a good amount of time to be able to spend a few days in each place to really get a full understanding of each attraction. For the larger places, you may need to spend a week in the bigger cities. However, you would really need to plan ahead of time each place and the attraction you would want to see in order to see everything in India within a month.
Touring India is a huge undertaking. Not only is the country large, but it is diverse with many options on where to stop and stay. If this is your first trip and you really want to get a feel for the culture, consider a longer trip—2 months should suffice. When traveling, you should always take into consideration the amount of time it takes to get to your next destination as well as how much time you'd like in each place.
Ideally, spending 5-7 days in each city, with longer periods on areas of focus or interest. Because India is so diverse, you should aim to travel both the northern and southern parts of it.