Ok. I know that there’s no right answer, but I’m curious about everyone’s opinion...

If you had to pick one length and stick with it for the rest of your life, what’s the best sling length for target practice with tennis balls?

C_A has some specific way he works out a good length for slings. I've found the ones I got from him to be the perfect length for pretty much all styles.

I use street hockey balls rather than tennis balls.  They're basically the same size but just slightly lighter in weight and don't have any fuzz.  The weight is usually around 50g.  The sling I normally use when I'm throwing them around the yard is 28.25 inches (72 cm).  But when I'm throwing at a particular target (usually with ice balls) I tend to use a slightly longer sling.  The one I've been using the most in recent days is about 31 inches (79 cm).  If I had to pick only one length to use from now on, I'd probably go with the longer of those two.  So my choice would be around 31 inches.

Wow! Not a ton of responses, but I expected shorter lengths than that for target practice.

I do have a couple Balearic slings that I use occasionally.  Don't get me wrong, they're definitely fun.  But I always end up getting back to a sling that's in that 30ish inch range.  I find that they work well enough for target, and launching a projectile for distance.

@Kick - Do you have a length on any of the slings you got from CA?

81cm from the centre of the pouch to the end of the release cord. I've found this to be the perfect size for me.

   There are many good slings for tennis balls.  My favorite is the Seatbelt Sling.
Edit:  Sorry, I misread the question.  For me the ideal sling length for tennis balls or anything else is half of my height or 38".