Over how many years does the "Song of Fire and Ice" series take place?

I was curious as to how many years George R.R. Martin's "Song of Fire and Ice" series takes place. No spoilers please, I've only seen the first season of "Game of Thrones" and am barely 180 pages into the second book. I was just curious because the producers of the show said that with the lengthening of each book the T.V. series could potentially last ten seasons. If this was the case and the series does not take place over as long a time period, it would seem to me that many of the actors would out age their characters. I know, its an impressively nerdy and pointless question but I am very curious.Over how many years does the "Song of Fire and Ice" series take place?
Though it has taken decades to write, and is still a work in progress, the Song of Ice and Fire saga to date spans not more than a couple of years.

I have been reading it since it first started and the 4 to 5 year wait between the release of each new book is a source of great frustration, but you have a great advantage, being able to read the first five volumes without having to wait.