Here's a post on how Leo was able to fully heal for the final movie scene.
Oh YEAH. It is NOT scientifically inaccurate for him to be walking around so shortly after HELL.
("so shortly" standing for 7 to 12 months,, New York had the time to rebuild and they apparently spent this whole time pizzaless until the "grabbing a slice" scene with Casey Jr)
First of all, the guy has at least one concussion. A bAd one. That's two to four weeks for sure.
Pretty fvcked-up arm (I chose his right one, like Future Leo). That's three months in a cast.
Now, you know how turtles' shells are literally their ribcage? I figured breaking their plastron/shell would be similar to breaking ribs. And just as painful. But that's just a detail.
I'm not kidding, you ALL saw the Kraang punching his chest repeatedly full force. There's no way his plastron didn't give out, it caved in a bit.
So I'd say it did some inevitable damage to his lungs and heart. I picked temporary arrhythmia (it happens when there's an issue with the heart's tissue), that's like, really fast heartbeat at random times of the day for no reason. Tachycardia attacks feel like panic attacks, except it comes with chest pains and you can FEEL your heart going haywire.
And his shell didn't survive BREAKING CEMENT AND ROCKS either. It's a little less severe than his plastron, he'd definitely lose one or two scutes (they grow back... kinda. Just pretend he was about to shed anyways).
Turtle plastron and shell fractures might actually take three years to heal, BUT, he's a mutant, and human broken ribs would take about three months.
Let's just round it up to six months.
That would be at least one month before the ending scene.