After being released on a Chapter 18, how long do I have to wait before I can rejoin?

CPL Garduno -
If you are still on active duty, you can seek guidance from either a career counselor or a JAG Officer.
I'm sure they would ask you why you couldn't have controlled your weight while you were in a unit and what makes you think you can keep it off once you get it off should you be allowed to return?
These are questions you can reasonably be expected to have asked of you so be prepared to answer them. But the rules are the rules and irrespective of the current circumstances or future expectations, they owe you an accurate answer.
I hope this helps and wish you the best no matter how this plays out.

Typically 6 months.

You think it will be easier to meet height and weight standards once you get older? Once you've been a civilian for years, no one pushing you to do PT, working full time and no free gym membership? You have all the keys to success laid out before you right now. You get paid to do PT, free gyms, free dietician, free fitness professionals on your post or even unit. You just have to do the work. It doesn't get easier when you leave the Army. I got out of the Army and I sure didn't keep up on my PT and I was 21 and healthy.

The Army is going to make you wait a few years and then they're going to require a waiver. That's how long you have to wait - until someone in Recruiting command believes you've dropped enough fat that you won't get kicked out a second time. Until someone you've never met and never will meet is convinced that you won't be a liability and waste of time and money for the Army and a headache for your next command.