Im sure its based on the same as real life, but is devided by the race time/laps you select for a race.
In English.....A race is 500km OR 3 hours, whichever is sooner.
a 60 minute race is a 3rd of 500km, = 166.6666666666666666667km
Monza circuit length is 5.793km (in GTR it's 5.77km (5.770m))
Which works out at 28.88 laps
IF Each lap took 1:45 then the race would reach its 166.666666etc... distance at 50.54 mins.
If the race went the full 1 hour,you would end up doing around 33/34 laps and in turn would then do more than the distance required in the rules.*
<puts away calculator>
*I Think.
........................................................awaits for someone to say its all wrong