Welcome to Thunders.
This works differently for everybody. It might take somebody 3-6 months to gain an inch; others may take a couple years, if ever. There’s really no way to tell until you try. Two inches in length may not be too much of a stretch, but the two inches for girth is a bit lofty. Not that it hasn’t happened. As you read through these forums, you’ll find some big gainers. I didn’t start seeing any gains until about the 6-7 month mark.
But just understand that this does take a lot of time, dedication and patience. Having the goal of 8x6 is nice; just don’t allow your goal to set you up for disappointment. Also, take you time in the beginning. Follow the Newbie Routine. Too many people — myself included — try to do too much too fast, and end up with injuries.
Good luck.