Its in the track gdb file, you will find a list of the session times, just backup the file and edit the lines to get the lengths you want. Here's an example of the lines you might find in a gdb, and you can add or change any of them.
FormationSpeedKPH = 100
RacePitKPH = 60
NormalPitKPH = 60
TestDaystart = 14:00
Practice1Day = Friday
Practice1Start = 12:00
Practice1Duration = 60
Practice2Day = Friday
Practice2Start = 14:00
Practice2Duration = 60
Practice3Day = Saturday
Practice3Start = 14:00
Practice3Duration = 45
QualifyDay = Sunday
QualifyStart = 12:30
QualifyDuration = 60
QualifyLaps = 12
WarmupDay = Sunday
WarmupStart = 13:30
WarmupDuration = 30
RaceDay = Sunday
RaceStart = 14:00
RaceLaps = 20