You don't need to clear out the render cache. Once you reach the limit set, Poser overwrites the old ones. They don't just keep piling up. You can leave them alone.

Attached you will find a script I wrote that gives a lot of info about your render. It includes the total running time of the render. Just run it, and it will render your scene from the script. Make sure you remove the .txt from the name!

Hope this helps!