How long will it take for India to surpass China's economy, considering their advantages in democracy, better relation with the west, etc?

It is mentally unhealthy for India eyeing at China. If India can make its people happy and it's economy prosper, passing China or not is irrelevant. The priority of the Indian government should be developing a prosperous India, not passing China.

If India surpasses China, then what's next? Surpassing the US?

When India got independent in 1947, it had much better industries and infrastructure than China had at that time. If India has so many advantages over China, there should be no question that India should have surpassed China. India surpassed China in 1947. So the question is not how long India can surpass China. The question should be since when India had lagged behind China.

Buddism propagated from India to China. China learned from India and adopted the attitude of Buddhism. China has had never the goal of surpassing India since 1949, yet it surpassed India somehow. A country should be more mature than an individual. Eyeing China as India's national agenda is very childish.