AO - based on personal experience and the usage levels of my using friends, 40mg would be what I would consider a light habit. In fact, I don't know anyone who was using for years that didn't escalate to a higher level than that. To put it in perspective I have seen folks get to >1g per day of oxycodone. Also, there are lot's of folks that take pain meds for legitimate reasons that are prescribed doses that are multiple times what you husband is taking. I wouldn't characterize that dose as a 'ridiculous' level at all.
All of that said, coming off of even a light opi habit sucks. Based on personal experience, if someone has been using daily for years they are going to get some withdrawal regardless of dose. The fact that he was only dosing once a day seems to be a positive. In my case, I would be fairly sick if I had to wait a full 24 hours between doses. If he is able to get away with having withdrawal for only a few days and then being OK then he has it relatively easy. It sounds like he doesn't have a choice given that he can't get any more for the next 3 weeks.
It sounds like he came clean to you and admitted that he had a problem. Also, he flushed the rest of his pills, right? What else are you expecting from him at this point?