www.pro-football-reference.com says Manning's 19 (9 or 10 is an average year) puts him at 254 in Career Approximate Value (ties with Brett Favre) and 172 in Weighted Approximate Value (13 points above Jerry Rice and Ray Lewis).
How hard is it to post a 254? That is 15 seasons at almost a 17 pace. So basically a high end Pro Bowl player challenging for MVP every year for 15 years.
How hard is it to post a 172 Weighted? Well, Jerry Rice only got a 159 and he was great forever. Or looking at it more mathematically, let's say you had 20 points a year for 20 years (you were the MVP every year you played for 20 years).
You would end up with
20+19+18+17+16 etc.down to your 20th year, which would be worth 1 point.
that only equals 210 points. Manning has 172.
Barring some guy who comes into the league and starts putting up 30+ seasons (example would be you rushed for 2,500 yards and 35 TDs while the rest of the league had no one above 1,000 yards or 12 TDs), it seems extremely unlikely that anyone is going to approach 254 any time soon, and the sustained excellence of having 15+ high end Pro Bowl seasons seems unlikely as well.
For those of you looking out for Tom Brady - he posted a 13 for 2013, taking him up to #14 on the non-weighted list and #8 on the weighted list. This moves him up from 16 to 11 on my all-time greatest list, ahead of Rod Woodson and just behind Lou Groza and John Unitas.