That would depend on how big your window is and what orientation it will be in. You should be able to get away with 3 mm just fine, if the window is in the range of a regular tower. Make sure that you don't get cast plastics, they are not a flat surface and will refract the light. If you tell us the size and what it is going to be used for I can tell you the minimum thickness you can use. If you have a drawing with dimensions I can give you a pretty picture telling you if it will break or not (FEA).