Replaced O2 sensor, gas mileage slowly coming back

So I replaced my upstream O2 a few weeks ago. I only drive about 12 miles a day on average, so a tank lasts me quite a bit. I didn't see much gain off the first tank but when I did the next fill up (half tank technically) the numbers started looking better.
Does it take a while for the ECU to acknowledge the new 02 sensor? If so, what have you guys seen for timeframes?

 

It took me about 7 start ups over the course of a few days before the ECU relearned itself.

 

You can reset the ECU by pulling the EFI fuse for 15 mins.

 

Just replaced mine a few weeks ago. Haven't really seen much MPG improvement, although that could be due to my lead foot lol. Usually with that kind of stuff, it takes a couple of hundred miles to properly re-learn itself. Might be less if only driven on the highway.