OMPM time estimates

I am looking for roughly what the size an organization would have to be to make using SQL express no longer a viable option? I am thinking of table size limits in SQL Express and those kinds of things, or the sheer number of rows that could potentially be generated when doing an large assessment. Lastly, I have not been able to find any general guidelines as to the length of time the three different scans could take. I am trying to come up with some time lines and running it in my lab, isn't really indicative of the real world in that way. I know that there are too many variables involved to give a hard answer, but are there any "guidelines" such as: For every 500,000 documents scanned Lite scan = x hours Deep scan = x hours Macro scan = x hours Thanks! ...