there is NO estimate..

it depends on type of light how many lights what kind of scene how many items are in the scene does it have shadows and how many shadows are there..

what texture size is used on the items... is there any light FX like AO is volumetrics used 

Etc etc etc etc etc  ...

So lets say it wil take TIME.. just try it out and fill up a scene with things that you need then count and add time from that until you know what you need and time of rendering..

 

 

Chris

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