I understand that whenever I create a Global Secondary Index (GSI) for a DynamoDB table it will take some time to create that GSI (depending on table size).
From what I understood, loading the items from the base table to the GSI only consumes the WCU of the GSI.
Let's assume I have a DynamoDB table with Terrabytes of data in it. if I create a GSI with 1 WCU, how long will it take for the GSI to be created (if all the items and values have to be projected)? Could it be high values such as multiple months ? (the doc states it takes around 5 minutes)
It could take a really long time yes. You'll be writing 1 item per second, so if you have 1 Billion items it will take 1 Billion seconds to create.