As someone already posted, there is 9k XP cap which means:
x5 multiplier - after gen 8 you gain nothing besides heirloom (1800 XP per minute without multiplier)
x4 multiplier - after gen 13 you gain nothing besides heirloom (2250 XP per minute without multiplier)
x3 multiplier - after gen 20 you gain nothing besides heirloom (3000 XP per minute without multiplier)
x2 multiplier - after gen 35 you gain nothing besides heirloom (4500 XP per minute without multiplier)
x1 multiplier - after gen 80 you gain nothing besides heirloom (9000 XP per minute)
In order to achieve average XP gain per minute of 9000 XP, one must win average 50% games and have many generations behind him (I'm not sure how to calculate this but I think it's beyond gen 30). Being that high generation means that it'll take him a lot of time to play as gen 30 and above character (I'm not sure but my crude estimate is that's more than 200 hours put into that generation only) to achieve infamous lvl 35 which is if I'm not mistaken around 140 million XP. If we take into account that average c-rpg player will need two weeks to get to lvl 31 and retire which means average gen one or two player will need 60 weeks of WORK to get to gen 30 and 60 weeks equals 420 days or 14 months! Going from gen 16 (I believe that's Kesh prepatch) to gen above 30, even if he retires every 4-5 days will take him 56-70 days which is about two months. In that time chadz will release another patch which will fix this issue if needed and nulify Kesh's hard work.
I haven't done any serious math in a long time, I encourage those who are better than me (most of you I suppose) to try and create correct model which will gave us all precise numbers.
EDIT: I need to correct myself. I forgot to add a month or more for imaginary Kesh to get to 140 million XP (if what I've written above is near to being correct). Also I heard but I'm not sure about it that it still takes 7 days to retire regardless of amount of XP and lvls someone has. If that's true that's 7 days x 14 gens = 98 days + 30 or more days = 128 days which is more than 4 months for imaginary Kesh to get to lvl 35 if I'm correct in my calculation.
Also, lvl 35 isn't gamebreaking as most of you think it is. Before the big patch me, being a lvl 31 was perfectly able to kill players who were lvl 43 and above. That's 12 lvl difference. Now that's just 4 or 5 lvls. People who are more skilled than me had no problem killing me before the patch (and being lvl 44 like Ujin was if I remember right) and they certainly have no problem now, even if I'm lvl 30 and they lvl 25. Being higher lvl can help a lot only if their skill if very low but even then it's not hard to kill them.
And if you're worried that someone about your skill lvl will have an advantage over you because he played longer than you and therefore you want to take that away from him, in that case you're an ass. Everyone of us choose how to spend his god given time, if he wants to play more c-rpg that is his problem not yours and he needs to have that slightly edge over you. That advantage was solid before, now it's all about different builds. Being lvl 35 will put you slightly above those of your skill lvl but won't make you twice better than them. Don't forget my point is that getting to lvl 35 takes too much effort. Since I heard that you're lazy musician Gorath, I wouldn't recommend to that to you.