Assuming he's been using the full 6 hours since age 18, that would only give him an extra decade on Snape.
We're not comparing Dumbledore in his thirties to Snape in his thirties, but Snape in his thirties to AD at "64". If we assume that he's been using his time turner since age 11, like HP (though based on his backstory, it seems he got a large intelligence boost at 10, so that might be where he started), he's effectively over 77 when he fought Grindelwald, giving Snape upwards of 4 decades (longer than he has lived thus far) to reach Grindelwald-defeating levels of power. In addition, we know that Snape has good reasons to hide how powerful he is (...
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: