How long does it take for precancerous cells in your body to become cancer?

Varies: You start acquiring the mutations that produce cancer when your life starts. Cells that a pathologist can tell are "precancerous" signal "time for treatment". Cases in which the mildest of dysplasias of the cervix have turned to invasive cancer in less than a year are well known. Others simply disappear, or never progress. Be glad for early detection -- and congratulations on a likely cure.

Maybe never: Every cancer has it's own growth rate which we define as it's "doubling time"(how long does it take for the cancer to double in size). Some pre-cancerous cells never have the ability to transform into invasive cancers, however that is probably the exception rather than the rule. Was there a specific type of cancer you were referring to?