I think it depends on how low you were to begin with.

Some people are running only on liver stores. Others are absorbing some but not ideal amounts from food.

The body will want to use the B12 in whatever priority it seems to have. Some will get stored back up in the liver. Some will pass thru the blood brain barrier into the brain and spinal cord, some will go to the bone marrow for making new red blood cells, and then other tissues will get some too. I don't think anyone has discovered the priority issues, or tested them. But like every thing else, we are all unique and will respond the way the systems determine inside of us.

I think 3 months at 5mg a day, is enough to show if things are working well. If you stop the B12 for about 3-7 days, the last dose orally will be soaked up and the blood level will reflect what you need. If you test really high that may show your systems are "full" and the excess is floating around waiting to be excreted. If you are not really high (over 1000), then you are still repairing what needed it most (or you are not absorbing it properly).