MR11 - What is the optimum number of entries to be processed in one run?

A client has been live for some time now but has never run MR11 to clear Goods Receipt entries without Invoice. They are nearing year-end and have to clear all those entries as fast as possible. Problem is MR11 keeps timing out after running for several hours, despite increasing the time limit. Has anyone run MR11 to process mass postings. If so, what is the highest number of entries?