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I would say that Radiance is too 'sophisticated' for what you want to achieve here. It has quite a steep learning curve. If I understand correctly you want to obtain the fraction of a window's surface area that is exposed to direct solar radiation.

Some tools/methods with which you can obtain this:

Ladybug (plug-in to Rhino/Grasshopper or Revit). The sunpath/sun expore components.
EnergyPlus. Request the 'Surface Outside Face Sunlit Fraction' output for a particular surface.
With Radiance this actually becomes a bit more complicated. I would recommend the two phase method (Daysim). You would have to setup a grid of points across the window. Measure Irradiance/illuminance and count fraction of points exposed. You either have to use a self defined illum/Irrad threshold above which you consider the point exposed to direct sunlight. Alternatively you could request only the direct radiation and count points > 0.

Some tools/methods with which you can obtain this:

Ladybug (plug-in to Rhino/Grasshopper or Revit). The sunpath/sun expore components.
EnergyPlus. Request the 'Surface Outside Face Sunlit Fraction' output for a particular surface.
With Radiance this actually becomes a bit more complicated. I would recommend the two phase method (Daysim). You would have to setup a grid of points across the window. Measure Irradiance/illuminance and count fraction of points exposed. You either have to use a self defined illum/Irrad threshold above which you consider the point exposed to direct sunlight. Alternatively you could request only the direct radiation and count points > 0.

Climate consultant can also give you some idea of shading by obstructions and shading elements. Not as an exact fraction of for each minute though. The input is extremely easy though.