I realized that I did my previous ledge invincibility testing in Training Mode, which may not keep track of Air Time, number of ledge grabs, etc., so I decided to try testing in Versus Mode (or whatever you want to call it).
Just played a 5-minute game as Jigglypuff against a Level 9 CPU Mario on Battlefield where I did very little outside of stalling in the air with jumps and Forward-B. I took about 60% damage the whole time, and I won the match (killed him once; he slowly but surely took damage from Pound and rising aerials). Most of the time I refreshed ledge invincibility by expending most of my jumps and airdodging into the edge of a platform, so I was on the stage for an incredibly short amount of time, and I tried to vary the amount of jumps/time between ledge grabs a great deal. There was at least a full minute where the only button I pressed was Jump, just in case it ran on the "Drought" statistic, as well.
I didn't notice any significant differences arising as a result of anything but damage. Even at 40-ish%, and four minutes into the match, stalling in the air for about ten seconds and grabbing the ledge for the hundredth time gave me about 60 frames of invincibility.
EDIT: Did more testing, unhappy with the results. Turns out that Sakurai was right about Air Time affecting ledge invincibility, but it was in the opposite way I expected it to. The longer it's been since you touched the ground, the more invincibility you have when you hang on the ledge. If someone at 150% runs off the stage and grabs the ledge as soon as possible, they have the minimum amount of ledge invincibility of about 20 frames (if you buffer a ledge option, you're still not vulnerable before that action happens).
If you get hit by a strong hit to the blast zone or just stall for a couple seconds, you gain a lot more invincibility; seems like it's up to about an extra 60 frames, depending on how long you're in the air. A Jigglypuff at 150% can stall with a couple jumps and a Pound and have over a full second of invincibility, while doing the same at 0% lets her be invincible for literally two whole seconds.