Was Buddhism the dominant religion of India for one thousand years?
Wikipedia's History of Buddhism in India says that:


Maurya Empire (322 BCE–185 BCE) was Buddhist
Kushan Empire (30 CE–375 CE) was Buddhist


Decline of Buddhism in India says that decline began in late Gupta Empire (320 CE–550 CE)

So for more than 500 years, there were Buddhist empires ruling the whole of what's now India except its southern tip.