How long after stroke does dementia appear in those who will get it?

Varies. Every stroke has different outcomes, depending on the location of brain and the type of stroke (blood pressure related, blockage of vessels, blood clot, bleeding, high cholesterol, etc). The development of dementia could begin immeidately, few days or few months after stroke. It is important to start cognition retraining immediately after stroke if this person is fully awake.
That depends on. The underlying condition of the brain. If there is evidence of less dramatic problems it can happen at any time after the stroke.
Immediate. After a stroke (cerebrovascular accident of various types), the neurological deficits are essentially immediate and maximal within a day after the stroke. Dementia (significant loss of cognitive function) may likewise occur immediately and abruptly after a stroke, distinguishing this type of dementia from other types. Generally, recovery occurs gradually afterwards for up to 1 year or so.