I haven't started our campaigns to capture DOI consent yet (scheduled next month). My gut reaction is that the conversion rates will be VERY VERY poor for prospects. Reason being is the number of people who will want to confirm opt in now when their initial interest was 3 months ago (or even later) is not very likely. Even then, this is based on the assumption that they saw your email and opened it in the first place (looking at your open rates will give you a good potential DOI idea).
With the GDPR coming, I feel tons of marketers will suffer from the same issue which can be viewed as bad, or good if you care about having an active and clean database.
I really look at this as an easy way to do some house cleaning....
Not an answer, but my thoughts. I'll follow up after launching our campaign to let you know.