Four years. I've had my helmet for four years. In that time period my helmet has experienced a lot of sweat; however, in that time period I haven't cleaned my helmet once. Yep, not once.
Recently I noticed that the stench of my helmet had gotten out of control. It needed to be cleaned and pronto.
So here's what I did:
After the ride, I took my helmet in the shower with me. Making sure to grab an extra hand towel before I stepped in.
Once I was sufficiently clean, I turned to my helmet and gave it the exact treatment - put some body wash and some shampoo (just for the heck of it) on the towel and began to scrub my helmet inside and out. Once it was thoroughly washed, I followed the shampoo instructions to "rinse and repeat" and gave it another thorough scrubbing - this time making sure to focus on the helmet straps.
Then I hung it up to dry.
The next day my helmet was as good as new!