I've heard velogirl's rule of thumb before, and find that a lot more reasonable.

I haven't been road cycling for a year yet. I had a gradual build up from a long ride of 30 miles, one day a week (with shorter rides the other days) to 375 miles one day/week (actually, that took two days!!) over a period of seven months. Since that longest ride, I have scaled back to much less total mileage, and shorter long rides. I will begin to build up again in January, starting with a base of long ride of 125 miles as my longest ride per month, with about a 200 mile/week total. I don't ever want to fall below being able to ride 100 miles one day/week if I want to.

So, even though I put in a lot of miles/hours- I'm still a newbie in terms of years of road cycling!

The article talks about what percentage of your weekly mileage the 100 mile ride is. If you're putting in 200 miles/week, then, it's not as stressful as if your weekly mileage is 100 miles a week. (But, if you're in the process of training properly for a long ride, hopefully you'd be doing a 70-80 mile ride as your long ride of the week, with three-four days of shorter rides.)