The Walk into Mordor blog has made a valiant attempt to compile a list of the longest fantasy series. The author's criteria was to count only series over 2,000 pages (thus avoiding the vast glut of trilogies), but based on reader feedback even with this criteria his list has still swollen from forty to over seventy series, and still rising based on feedback.
The list's counting methodology is somewhat arbitrary - the heavily interconnected Belgariad and Malloreon are counted as two separate series whilst the mostly stand-alone Discworld books are counted as one - but nevertheless it is a fascinating list. The fourteen-volme Wheel of Time series (counting the prequel spin-off) is actually considerably longer than the thirty-three volumes of Piers Anthony's Xanth series, whilst Discworld's place in the lead isn't that much higher considering it's forty books written over thirty years compared to fourteen over twenty. A Song of Ice and Fire's placing at #32 (at the moment) is surprisingly low, but Steven Erikson's ranking at #7 is definitely surprisingly low, until you realise that it's the most recently-begun series in the top seventeen, showing his rate of output to be quite impressive.
Interesting to see how the promised SF list is compiled and how dominant the likes of Peter F. Hamilton are, although I think the forerunner at this time will be the Perry Rhodan series (over 2,500 novellas forming very long and distinct story arcs).