US citizen- OCI - How long can you stay in India?

Correct, OCI is like a green card, with the additional benefit that no minimum stay is required. So if you do not live there, you do not lose your OCI.
 
How long one can stay in India after getting OCI (US citizenship) ?

no limit. many OCI holders are living in India and are not coming back to US. if you stay 5 years in India you can again get Indian Citizenship (if you wish). you are staying like just any other Indian citizen.
 
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