"in the standard repertoire", yes.
However, not the actual longest, which if we stick to the title "symphony" would have to go to Kaikhosru Sorabji's "Jami" Symphony (Symphony no. 3), which clocks in at around 4 hours, 30 minutes.
This is actually a symphony, not a filmscore, not some extended concert work, nor a piece of experimental theatre music performed in the forest overnight...
Kaikhosru Shapurji Sorabji
you will find this work listed on the McGill university website above, and it is clearly listed as a symphony. It is in 4 movements.
The timing I found has the 1st movement clocking in at 1 hour 34 minutes (the entire length of the Mahler symphony), the second movement nearing 20 minutes, the 3rd movement alone at 2 hours, and the finale just shy of 45 minutes.
Now, for all I know, the music is crap.. but then, Sorabji IS a known composer. His works have been recorded numerous times, as well as published.
He is also known for writing LOOOOOOOOONG pieces of music (his "Opus Clavicembalisticum" lasts just under 4 hours, and HAS been recorded a number of times (takes up 5 CDs).