For those of you with many many hours played, how many in-game years does that relate to?

3E stands for the Third Era.
Each era has ended after a significant moment in history, not after a certain amount of years.

You might want to read about the http://www.imperial-library.info/content/tamriel-timeline.

The calendar is similar to our own: http://www.uesp.net/wiki/Lore:Calendar

Lore gets derided a lot, but I just love things with enough substance to have eras. Love live Beleriand!

Yeah but don't the years change? In my game I am in 3E433 - I thought the 433 was the year of the era? I haven't really looked into it tbh!

Yes, the game takes place in the year of 433, but I can't remember if the year actually changes to 434 if you play long enough.
But the era wouldn't change anyways, since the game doesn't cover that stuff.

If you want to do the math, you can find out how long it would take to play the entire year.
1 minute real time = 30 minutes game time

It does. As long as you don't finish the MQ, it'll stay in the 3rd Era, but years can still change to 434, 435, etc.

The Oblivion game doesn't change Era when the Main Quest finishes, so most characters that do the MQ first find themselves in 3E434 or higher, instead of 4E1 etc.

My explanation is that it takes an Imperial decree to start counting a new era, and the event is backdated as appropriate once the new era is official. So the first year that's officially a 4Ex one might be 4E5 or something, with the already past years renumbered after they happened.

I just want to say that I knew the era wouldn't change it was just the year number I wondered increased

And if so what were people's 'highest' years