I had been thinking about this for a few days and finally sat down to do the math.
As of today, El Guerrouj has held the mile WR for 8 years, 1 month and 9 days (someone with more time can count up specific days). (7/7/99-8/16/07
Before him, Steve Cram also held the WR for 8 years, 1 month and 9 days. (7/27/85-9/5/93).
Jim Ryun complicates things slightly. His 3:51.1 lasted from 6/23/67-5/17/75- 7 years, 10 months and 24 days. BUT his 3:51.1 lowered his 3:51.3 from the previous year (7/17/66), about 11 month prior. If you add in this additional time, Ryun held the mile world record for 8 years and 10 months.
So, depending on how you look at it, El Guerrouj has either held the mile world record for longer than anyone already(in the sub-4 era), or will, if his record goes unbroken until May 7, 2008.
yes, i realize i should use total days to be totally accurate, but this math was already a stretch for me.