What you were told is a huge generalization and not really accurate. You see, everything depends on the hiring situation at the time and on your seniority when you make the change. For example, when I was hired at Continental a lot of pilots were hired on behind me, so I was only on reserve for for a month. About a year later hiring ground to a halt with the recession and the junior pilots were on reserve for several years until we started hiring again.
When you upgrade to Captain the time spent on reserve depends on your seniority. If you take the first possible Captain slot then you could be on reserve for years. Many pilots wait to upgrade until they can be line holders and thus never go on Captain reserve at all.
Once you have enough seniority to become a line holder you will generally not sit reserve again as long as you are in that seat on that piece of equipment.