I have not done any formal scientific testing, but I have determined through observation that the timing and quantity of spit-up directly correlates to the cuteness of the outfit being worn by the child and the niceness of the outfit being worn by the mother.
If the child is wearing an adorable outfit, the spit-up always occurs after leaving the house but before reaching the destination. This is particularly true if we are destined to see the person who purchased the outfit.
Again, if the mother is wearing a nice outfit, feeling attractive that day, and especially if a garment is dry clean only, the spit-up will come in great quantities. As for timing, it will happen after you reach your destination, about an hour or so before you notice it.