Hecate After years of research, I reckon:
Invited to dinner: be 10-15mins late (this is because I'm never quite 100% ready on time when making dinner for people, and neither are many others; however, much later and they start to worry).
Average house party: be about an hour late unless the host is a really good friend and might need help setting up;
Parents' generation party: be on time;
Filthy squat rave: be about 3 hours late. At least.
I still screw up, frequently. There needs to be a Book.