I don't think there's any such thing as "too deep" as long as you don't hurt yourself. And remember, there will be some days that you can, and some days that you can't. Just because you can take 36" one day doesn't mean you can the next day. Listen to your body and remember pain means stop!
Earlier today, I had a nice series of enemas to clean the playground and spent a couple hours playing with some of my toys. Finished up with the 1" X 24" solid colon toy from Silicone Nozzles. Initially, it dodn't want to make the turn into the sigmoid colon. I just sorta got in the zone, lying there, my hand running the toy and my mind just enjoying the sensations. I don't know if it's this way for everybody, but I get a very pleasurable feeling from something pressing or tapping against the end of my rectum. So, I just kept lightly pressing it forward, twisting it a bit from time to time, and eventually it just slid into my sigmoid with no problem at all.
Felt great as it progressed up the descending colon and I could feel the tip at the splenic flexure. But it wouldn't make the turn. Spent about a half hour in the zone playing with it. No joy. It felt great going in and out, but it would not make that turn. So, today I didn't get the retention balls in. I did a couple days ago. Some days you can, some days you can't.
I recognized that today was a can't day. I didn't force anything, and I enjoyed everything I did. Plenty of lube, lots of fun, I didn't hurt anything, and I won't be sore tomorrow. A mighty fine day.