If I study MBBS and PG in India and go to the US for USMLE, how long will it take me to become a doctor?

If you want to go to US, then my suggestion is to start preparing for USMLE from second year MBBS onwards. Write step 1 in your 3rd year. Prepare for step 2 in the final year and internship. Write step 2CK during internship. Apply for elective rotations or research also during internship. After internship completion, you’ll be ready to match in US residency. Indian PG is not recognised in US, so doing that will be a setback of 3–4 years, as you’ll have to do residency again.