There are different time periods for everyone. Some people who worked in medicine before hand are good day one. Anatomy lab does wonders for you. By the end of third year most people are pretty good with all this stuff. Doesn't mean we don't find it "gross", you just can hide it well. There are still a couple things that still bother me. They are mainly found under bad smell category. Some people are never good with some of that stuff and find specialties to match their sensitivities.
I know I can never forget some of the experiences of anatomy lab. It's just not natural. Though I chose not to attend many of the lab sessions, I think it's something that impacts you on a very deep level, and some people more intensely than others. I had trouble sleeping and eating for a while, but most of my classmates didn't seem to have this trouble. Even for me, the feelings quickly passed.I don't know about "gross" stuff. But things like pain and suffering (both physical and emotional), human cruelty, and even death...I'd say it happens frighteningly quickly.