Where is the timer if any on a GE refrigerator GSH25JFXB CC ?

You can find the defrost timer in two places: on the bottom, usually at the front, and near the thermostat. Here are some manuals: https://dl.dropboxusercontent.com/u/...zer_refrig.pdf
https://dl.dropboxusercontent.com/u/...ile_Artica.pdf
https://dl.dropboxusercontent.com/u/...ide_refrig.pdf
Some have an electronic timer and if it will not defrost the things to test are the element and the defrost thermostat wired in series with the element. Replace both when one is bad.