You can filter observations for date uploaded, I don’t know how to find out which period was the start, but at least you can see when first observations were uploaded and with what rate.
I can think of a few ways to view observation data for India on a timeline.
Within iNat, you can see a History chart on each taxon page – there is a filter in the upper right for location. The limitation is that the highest level is Kingdom, so you can’t see all observations at once.
GBIF has a fairly nice interface for charts/metrics, though this obviously excludes iNat records not sent to GBIF. (The India Biodiversity Portal also has a GBIF dataset that you can use the same tools on for comparison.)
IBF - India Biodiversity Portal began in 2008 and is supported by the top “institutions” – Inaturalist on the other hand has not had the same “institutional” support in India - so to me it is pretty amazing that in slightly shorter time span the organic growth of inaturalist has been phenomenal
While not meant as a critique of IBF - their tools are much harder to use and not meant for the “normal” user - by normal I mean a person who has no grounding in the biological sciences.
Many of the top users on IBF are also some of the top users / contributors on inaturalist India - but I suspect there are some mutually exclusive populations as well.
I find it interesting that September 2020 was the month that saw the most observations - it surely must do with the Big Butterfly Month India project