As wel all know, athleticism is the first thing to go in most great players. What will happen when years of carrying his tema catches up to him, what kind of player will he be? And when in his career will this likely happen?
This is actually an interesting discussion. Most players peak at 28-29, and you can start to notice a decline at 31-32. So going by the averages, Lebron will still be as dominant for another 5-7 years (he is only 25 now).
But there are four things that make this a unique case study:
1) He is only 25, but it is his 7th year in the NBA. He has played in the playoffs for 5 out of 7, never has played less than 75 regular season games, and played on the USA team almost every summer. I would argue that he has played very close to the most official basketball games out of anyone in the NBA over the past 7 years. So although he is only 25, he has the 'wear-and-tear' of a 29 year old (especially if you consider most players played in college for a couple years, which is much less grueling of a schedule.) So based on experience/wear-and-tear, he should be at his peak now.
2) He is an athletic and physical freak. Love him or hate him, you cannot deny that he is the most unbelievable physical specimen we have ever had in the NBA. (And it's not even close). We really have no way of knowing whether his rate of decline is anywhere near the average player. Isn't it completely possible that his physique extends his 'peak' capability by a few more years than most players?
3) The guy is a rock when it comes to injuries - likely a direct result from his physique. If you pay attention, his ankles are the size of most people's thighs. The few times he has twisted his ankle, he has never sat out more than 4-5 games. Never has had a serious injury since he came into the NBA. He seems to be immune to injury (just consider how big of a story this slightly - if at all- injured elbow has become?). Nagging injuries, and bad knees/ankles/legs/etc are a huge factor in determining the NBA lifespan (see KG, Brand, J. O'Neil, Oden, Hill, Hardaway, etc.)
4) Development of a post game. People with dominant post-games can still remain useful well-past the decline of their athletic ability. If Lebron develops a potent post-game (which I believe he will over the next few years), he can easily remain a force in the NBA for the next decade (and could likely play until he is 40ish).
So really, I think it's impossible to predict how many more dominant years Lebron has in him. But his long-term lifespan likely will come down to his ability to adapt his game in response to diminishing athletic ability (and making a permanent move to PF).