How many are "a few"? [Details inside].

This question came up after I wrote someone that I would contact him in a few days.
Or in one of these coming days.
Could there be global consensus about how many days it should be in order not to breach the promise?
Or is it personal, and depending on the relationship one has with a certain person?
After how many days would you call, and after how many days would you think that the call to you is probably not coming anymore?