Normally when we eat animal protein, the protein is broken down by acid in the stomach releasing the microgram amounts of B12 present. Then special cells in the stomach called parietal cells make a carrier molecule called intrinsic factor. This latches onto the B12 and when it reaches the small intestine, intrinsic factor then carries the B12 into the bloodstream (B12 is a very large molecule and doesn't do this well by itself.)

Some people develop autoimmune difficulties where the cells that make intrinsic factor are damaged by antibodies. With no intrinsic factor to help then, no B12 from food gets thru. The older name for this condition is pernicious anemia. Also if there is no acid to digest the protein, then B12 is not released to any intrinsic factor that may be present.

So if you have intact intrinsic factor, your B12 in supplement form may be enabled for absorption by it. If you have a failure with intrinsic factor, then passive absorption is the only way for the B12 molecule to pass into the blood stream. To do this it has to be present in high amounts in the right place. A timed release product will slowly release small amounts of B12 over time, and restricts this passive absorption potential. An immediate release or sublingual type will release all of the B12 into the saliva and stomach contents quickly. Timed release is not a satisfactory method for a complex absorption requirement for a molecule like B12. I don't know why they even make it.

Taken on an empty stomach, every day in amounts of 1mg or more, you can see improvement in blood levels within a month.
Typically 3 months is a better time period. Each person is different and rates may vary.