The Walk into Mordor blog has put together a list of the longest SF series as a companion to its earlier list on fantasy series. German SF character Perry Rhodan is by some considerable margin the winner here (a conservative estimate of 120,000 pages across hundreds of novels and thousands of short stories and novellas published over half a century). Marion Zimmer Bradley's Darkover series is currently running a very distant second (although the compiler hasn't yet counted C.J. Cherryh's Alliance-Union books, which will probably be similar in length).
An interesting project. Reader feedback almost doubled the length of the fantasy list, interesting to see how this one goes.