Is 10 days enough for India?

Only if you stay in one specific region, like Rajasthan or Kerala. Trying to see the whole country in 10 days will result in spending 40% of your trip in transit.

When is the best time to avoid the heat?

November through February offers the most manageable temperatures. Avoid April to June unless you are heading to the high-altitude regions in the Himalayas.

How Long to Spend in India

Spend at least 14 days for a first trip to cover a classic route like the Golden Triangle. If you want to include South India or the Himalayas, budget at least 21 to 28 days to avoid spending your entire trip on trains or domestic flights.

Assess your transit appetite. India is massive. Factor in that domestic travel between regions often takes a full day once you account for airport transfers or train delays. Don't try to cram North and South into less than 3 weeks.
Pick one or two regions. Limit your scope. Choose either the North (Delhi, Agra, Rajasthan), the South (Kerala, Tamil Nadu), or the mountains (Himachal, Ladakh). Seeing too much leads to travel burnout.
Pad your itinerary for reality. Always add a 'buffer day' every 4 days. You will eventually encounter a stomach bug, a train delay, or simple sensory overload. Build in time to just sit in a cafe and recover.

Is 10 days enough for India?: Only if you stay in one specific region, like Rajasthan or Kerala. Trying to see the whole country in 10 days will result in spending 40% of your trip in transit.
When is the best time to avoid the heat?: November through February offers the most manageable temperatures. Avoid April to June unless you are heading to the high-altitude regions in the Himalayas.