What % of actinic keratosis are pre-cancerous? How long would it take one to turn into cancer?

All and variable. Actinic keratosis by definition is precancerous, however not all lesions turn into cancer and there is variable rate of progression. Treatment of the lesion can prevent progression and development of cancer. Please note that some people consider actinic keratosis to be a form of cancer.
It varies. This is the precursor to squamous cell cancer of the skin. For this to become cancer sooner versus later, it would depend on the susceptability of the person and the exposure to sunlight and other factrors that cause it to become malignant. If a person has fair skin and is exposed to uv light and ionizing radiation, they are more likely to have any actinic keratosis turn into cancer faster.