At what point does the waiting time start from for F4 sibling sponsorship from India to USA?

Once you application was approved, you got a letter with your priority date on it.  Visas are given out in order of priority date. If 2005 priority dates are being processed in your category now, then you have about 10 more years to wait.

When a petition is approved, that means the relationship between petitioner & the person being petitioned has been verified, it is accepted for further processing & given a Priority Date (date of preliminary approval & acceptance for further processing. Date of submission of the original petition is not relevant. Processing, interviewing the person petitioned, etc, begins when visas become available for issuance. Over the past year, it has been taking 13-15 years for Priority Dates for sibling petitions for Indian citizens to become current & processing begin. Right now, it appears to be about 16 years. So, say your petition was approved & issued a priority date in 2017 - expect the person will get a visa interview around 2031-32. But since so very many Indians are filing petitions for relatives in India, chances of date regression are very high, so it could take a lot longer than 15-16 years. The person you sponsor must remain in country of citizenship until they have been processed, interviewed, approved for immigration & issued an immigration visa.

From once the application is accepted.....could take 20 yrs before it is processed