Approximately how long after a stroke does it take for brain tissue to fully decompose and does it appear with the same density as csf on a ct scan?

It depends: The size and type of the stroke, ischemic vs hemorrhagic, are the most important predictors. For most ischemic strokes, the majority of liquifaction necrosis (the decomposition) takes place in a few weeks but continued healing will continue after the necrosis. For hemorrhagic strokes, it typically takes longer for the body to absorb the blood and complete the healing process.

Months to years: But this depends strongly on the size, location and severity of the stroke and the presence of other factors that will drive the removal of cells. Consider that just because the nerve cells itself were deprived of oxygen and died does not mean that all the support cells also died and most of what we see on the ct scan are support cells.