I've been using wool exclusively for a year and a half. For us, usually the wool starts to have a urine smell to it, and that's when the lanolin is used up. If I just wash it and don't lanolize at that point it will work for a few days and smell again. So, I end up lanolizing any time the covers need to be washed. Most of mine are machine washable on delicate (Wild Child Woolies and felted Imse Vimse), and if I wash them in the machine it seems to totally strip the lanolin.
We have 6 covers that get used every day plus 5 occasional longies, Kissaluvs, and SBish knits that don't fit under all clothes. I have to lanolize the 6 workhorse covers about every 3 weeks.