Think of it this way... If we assume the average number of main stream releases in a season is around 20, each having at least 13 episodes, that makes roughly 104 hours of viewing. Include, now, at least 10 or so OVA or OAD releases per season, and that's an additional 5 or so hours. If that's roughly 109 hours a season, that means 436 hours in a year, or roughly 18 days of watching (non-stop).

Assuming that this type of demographic existed for the past 5 years, that would mean 90 days of watching. For the years prior to that, let's make a conservative estimate and say that total viewing time for shows released was 10% less for each half decade preceding the last. This means that from 2001-2006, you would have watched only 81 days worth of anime per year. Working back to as far as 1950, that gives us approximately 645 days worth of anime. Converted to years, that's about 1.7 years.

And this is a safe estimate, take note. This doesn't include all the doujin or internet streaming videos that float about places like nico-nico douga or Y! Japan, and it also doesn't include special releases during comikets and the like. It also assumes that you watch continuously, without any break. Lastly, this estimate assumes all seasons to have at least 13 episodes, or a quarter's worth. By all means, this is a conservative estimate, at best.

Given the variances, I wouldn't be surprised if the total viewing time may stretch between 1.7 to 2.5 years, but I'm too lazy to work on the statistics to back that up. As it stands, if you want to reasonably do this (without any restrictions as to how to obtain vintage anime, at that), then you might need to take at least 5 years to do it so that you don't die in front of the television out of starvation or sheer fatigue. Good luck with that.