I've gone without shampoo for many months now, maybe a year. I keep my hair very short anyway, so it doesn't matter very much. It hadn't even occurred to me to try going without soap. I could probably see it working fine now that I think about it.
If you have traditional-man length hair, you don't need shampoo and you especially don't need conditioner. Hair grows about half an inch a month, and your hair can only get so fucked up in the course of six months, so if your hair is within three inches long root-to-tip, fuck it.