Short answer: 403.4 out of a possible 1,261.6.
Long answer: Here are some stats for FBS teams since 1979 (when passer efficiency was first used), categorized by # of attempted passes.
--(Min. 12-24 atts.) 403.4---Tim Clifford, Indiana vs. Colorado, Sept. 26, 1980 (14 attempts, 11 completions, 0 interceptions, 345 yards, 5 TD passes)
--(Min. 25-49 atts.) 317.4---Bruce Gradkowski, Toledo vs. Buffalo, Nov. 1, 2003 (25 attempts, 23 completions, 0 interceptions, 435 yards, 6 TD passes)
--(Min. 50 atts.) 199.2---Chris Redman, Louisville vs. East Carolina, Nov. 14, 1998 (56 attempts, 44 completions, 1 interception, 592 yards, 6 TD passes)
In Division I its Colt Brennan from Hawai'i with 58 in 2006.
Riley Nelson for BYU. BYU @ Boise State. -4.5
27 touchdown passes
colt mccoy
Joe Namath
Colt McCoy
tom brady
Brett Favre has thrown the most touchdown passes in NFL history, with 508.
Colt has thrown the most touchdown passes in a college career
Colt McCoy
Tim Couch
Matthew Stafford (Detroit Lions) 2010
Marcus Dupree
he has thrown for six touchdowns on two occasions
Daryle Lamonica of the 1969 Oakland Raiders - six touchdown passes in the first half. Did not even play the 2nd half because the Raiders were winning 42-0 at half time. Lamonica could of had 12 touchdown passes if he had played the entire game.
Tim Tebow(461) Tebow didn't have 461 touchdown passes!