You say "Unless it brought your levels severly low and your liver needs to store more which can take some time."
You mean the deficiency brought the levels severely low, and after supplementation and subsequently stopping supplementation of B12 and other B-vitamins, the body could still need more potassium and magnesium to continue the "process" for some more time?
The "process" would be RBC production and powering the Na+/K+ pump? Both requiring potassium and magnesium to continue. This could go on for a while depending on our bodies.. days to few weeks maybe?