When I first started using the oils, I could feel my symptoms start to return in as little as 4 hours. I don't know how much half-life degradation it took to get to that point, but I'd bet it's basically gone within a day. @Johnmac's "30-60 mins half-life after very gradual delivery" fits with that.
On the other hand, my experience is like yours and I would notice something within 30 to 60 minutes. In my case this went on to give peak benefit about 4 hours later (rather than the depletion you mention at this point).
Do you still take 2 or 3 doses a day or have you settled into a less intensive regime? I found 3 doses a day started to give me overload and had to cut right back.