In my Application Management workshops I regularly ask people the following question. Say you have to replace somebody who's worked as a maintenance programmer on an application for several years. Assume that his/her successor has the same technical knowledge and experience but isn't familiar with the particular application. How long is it going to take to get the new guy up to 80% of the productivity of the current guy?
Most of the answers vary between 3 and 18 months, depending on the complexity of the application. Obviously the new guy will be able to do useful things with a couple of weeks, but the work is going to take much longer to do. People are generally happy to assume 6 months as an average duration. If you estimate the time the old guy needs to coach and transfer knowledge plus the extra time the new guy needs to do the work, you end up with about 100 man-days.
This is a substantial figure and certainly a trigger to think about how you could speed up this process and more importantly, how you can attract and retain staff in this knowledge-intensive domain.