Here are several properties and methods provided by the AVAsset class that may help:
- (void)_pu_setCachedDuration:(id)arg1;
- (id)pu_cachedDuration;
- (struct
{
long long x1;
int x2;
unsigned int x3;
long long x4;
})pu_duration;
- (void)pu_loadDurationWithCompletionHandler:(id /* block */)arg1;