i defrag once in 2-3 months, for me usually the game files get defragmented, not mainly because of gaming, but coz i mess with them
when the bigger files (800MB & bigger) get fragmented, i use a less time consuming method, just "cut-paste" it on other partition & then reverse-> "cut-paste" it to original location. this way it gets copied as a
contiguus file 
btw i use
DiskTrix Ultimate Defragger, its does the defragmenting really Fast.