We are in process of upgrading Sitecore 7.2 to 8.2 Update 5 (may be Update 6 later). As part of the process client wants to migrate the historical analytics data from SQL to Mongo (On-Premise). Client have huge analytics database on 7.2 containing approx. 3+ years of data in it by having multi-site, multi-lingual etc.
Although we are planning to leverage the Batch Aggregation by configuring the MaximumBatchSize and MaxThread in History Worker process. But I strongly remember that in my previous Sitecore Upgrade, it took days to complete the data optimization, conversion and rebuilding the reporting database, even when they had less bulky analytics database than this.
Hence wanted to check with the community if we have any ways/guidelines to size and estimate (may be ballpark) for the Conversion process with various factors effecting.