To be the greatest of all time you have to have the greatest numbers of all times.
The QBs:
-with the most SB rings have 4. Brees has 1
-with most passing yards: Favre-69,000, Marino-61,000, Manning - 50,000, Brees(30,600) isn't even in the top 20 yet(about 33,000).
-with the most completions: Favre-6,000, Marino-almost 5,000, Manning 4,200, Brees-2,700(Brunell is 20th with 2,753)
-with the most pass TDs: Favre-500, Marino-420, Manning-366, Brees-202, Steve Young is 20th with 232
These are just some of the categories and their leaders, and where Brees sits.
So, as you can see Brees, statistically is chasing Favre, Marino, and Manning. Montana was top 10 in each of these categories (9,9,10) with him being at the top in SB rings.
Since Montana is arguably one of the best if not the best QB of all time, I think Brees needs to finish his career in the top 10 in each of these categories and get at least 3 total SB rings. The higher he ends up in the stats sheet, the less SB rings he'll need. He'll certainly need to continue what he's done the last 4 years, for about another 5 or 6, in my opinion.