My father in law was told he had a mild stroke at some point, what are the chances of a major stroke and how long between the two usually?

Don't know: Minor strokes may/may not show up on definitive studies like mri, ct scan of the brain. Presentation, physical examination, residual deficit all help in making diagnosis. Risk of major stroke can be diminished by controlling blood pressure, evaluating/fixing carotid artery narrowing, controlling/treating cardiac rhythm abnormalities, controlling diabetes, controlling obesity.

Control risk factors: Many factors contribute to stroke. Some people have a mini stroke and never get a major one. Many do get a larger stroke later. Control the risk factors and do some prevention (no smoking, check and control blood fats, check for hypercoagulable state, check for atrial fibrillation, check for blockages in the carotids, take daily aspirin, "mediterranean diet"). Neurologist will be able to help.