If you move towards the restaurant fast enough within the first five minutes it seems you're good. But if snow or traffic, etc. makes that first five too slow, or not moving at all, and you get a warning it starts a 15 min timer and if you don't mark arrived within that time it's dropped. Despite them being able to see your location, and exactly how long it should take you to get to the restaurant, even if it is impossible to get there in 15, they start the exact same timer each time this happens. Yet, if you can jet in the first five it doesn't seem to matter how long it takes, I've even been stuck in traffic before. It's an incredibly stupid, unintuitive system. It is the definition of asinine.