I do not know the science behind the 6 month recommendation. Like Sue said biofilms are slow growing and maybe that's the reason. We live in such a consumable society and replacing things often seems standard accepted practice. And of course companies would like to count on future [annuity] business.

If you wanted to be fastidious then do as vendor instructions say. What I do is alternate cups a.m. and p.m. After soaking in hot tap water and dawn, and like you, pop in a baby bottle sterilizer where I use distilled water. I've measured the temp and it registers 212+ for over 8 minutes. Technically it's been 6 months for each cup but since I've been alternating I'll continue to use for at least another 6 months or longer. If any of the equipment starts to fail I will replace.

The bottom line it seems to me, when you replace depends on your disease state, I think. If you have MAC or other infection you might choose to follow vendor instructions for replacement for greater peace of mind, knowing you are doing everything you can to help to mitigate the disease.