FWIW, I can give one data-point; my lowest level of B12 (166) had been brought up to 1460 in Jan. 2012. Since then, I had a couple of more weekly shots, but stopped them in March 2012.

In August 2012, despite 2 more 1000mcg injections in June and July, the levels had dropped to 482.

Despite a complete alcohol & medication stop, that level dropped further to 375 by November 2012. Shortly after that, I started the oral 5000mcg methylcobalamin, so that by April 2013 it was back up to 1298.

All these tests were done in the same lab. The drop from 1460 to 482 was steep and unexpected, the further drop despite healthy living even more so.

In short, that's a 1000 point drop in 5-6 months.