You realy cannot rely on the "estimated" it is based on what the software sees immediatelyor the area it samples, in relation to the size of the image. 

That means that if the software samples a small protion of the bottom of the scene and it is very complex only half way up the image with a standard none complex sky it may overestimate.  Which would usually indicate a long time and then process in real time much sooner that estimated.  This can easily occur the other way around, short estimations and long processing times.

Proceduaral terrains and lighting are often the culprits that extend the processing times. 

A better method of spot sampling areas all over the image might give a more accurate time of processing.

jankeen.com