We don't know how long he will maintain being the wealthiest man on earth. But what I know for sure is that it will not be permanent. The position whatever it may be, is replaceable. Anything can happen any time. There's always a possibility that someone will dethrone him from the spot he is currently in depending on the perseverance of the person and the resources as well.
Always remember that the world is round. You could be at the top now, but there's still a chance you will hit rock bottom the next second. It may sound negative but it's true. You can always bounce back though. Getting up and doing better the next time is always a choice. After all, we learn valuable lessons each time we stumble and fall.
Only Elon knows if he will be able to maintain his position or not. Maybe for now, we can totally say that Elon is capable of sustaining his spot because he has the wealth and has variety of investments that keeps giving him huge returns. But there is no permanent in this world. Somehow, things will change when we least expect it. Even Elon i guess knows that.
However, if he can see things more in positive ways, and he can multiply his investments through rolling his money, then maybe in the future he will still occupy the spot for the richest man in earth.