At least 1/2 hr , but 1 hr is better.

The micrograms of B12 can be lost in food. Microgram dosing of drugs has been found to be severely impacted by presence of food. Esp fiber foods that breakfast often has. Micrograms is a very very tiny amount of anything. Because I know this from my training...I push the empty stomach factor. I have not seen any of the OTC websites picking this fact up for B12...and it has been over 5 yrs now since it was known for drugs. That just illustrates how medical info can get walled into little niches and not applied to other situations!

I had a patient in a clinic once whose doctor kept raising her thyroid dose. She was at .225mg which is really high.. and getting no response. Turns out she was taking it with Oatmeal!
She was a young woman with an intact thyroid...no Hashimoto's either. She ended up well, finally at .1mg where she belonged, after I intervened.