I've played this quite a lot as its a really solid and enjoyable game. But there are times when I'm way past the wave 60 mark (like wave 120 and beyond) and I'll still die because I can't hit the turtles.
I'd say 75% of the time if you pass wave 60, you can go endless. But for some reason there have been times where my fireballs just spam over and over and *MAYBE* 1-10 of my hits land on them. Its only when its a few left and chain lightning doesn't bounce, and if lava pools don't last long enough for them to stand in it from other turtle deaths.
So even wave 100+ if you don't have existing lava and can't chain lightning to them, turtles can easily reach you and just tick you down. Its the same hit issue that others have mentioned regarding the red worms. Just saying it can and does happen with turtles past 60 as well.