Refrigerator cycling. How often should it cycle on/off?

Answers so far are good. But you have to consider that even with proper care, that 15 year old unit isn't likely very energy efficient anymore.

I kept good care of mine, but decided to replace it after 20 years. Nice monthly electrical savings! I should have retired it earlier. (I just hated getting rid of something that still worked well, but I was PAYING for it.)

There are coils under some fridges with a fan that on some pulls air in from the front bottom. These can get very clogged especially if you have a pet. The grate on the bottom front can be removed and vacuumed using a crevice tool. I backed mine up to the outside back door and blew it out using a leaf blower. It is amazing how much dust and hair (fur) can accumulate. If you have this type and can get and keep all the coils very clean you will see a noticeable difference on how often the compressor cycles. Also make sure you check the door seal, they tend to deteriorate on the bottom. If your seal is bad you might just as well leave the door open.

HVAC Tech.: How often the unit cycles will depend on how often you open the door(s). Your defrost timer will only defrost once a day as it is a 24 hr. clock. Sounds like you've taken good care of it!

pull it out and clean the coils on the back they maybe full of dust and dirt.

but your fridge is pretty old , it shouldn't run for hours mine only runs for a few minutes.