A piece of mail can go coast to coast or any location in-between in 5 days. Sometimes a little quicker, sometimes a little slower - after 7 days it's about 99.5% that the buyer did not mail payment when stated and it is time to follow-up. Which usually earns some sort of lame excuse, lost my emails, gf dumped me, I forgot, uhhhhh, etc.... Sure thing Beavis. Trying to be patient with one now - see how it goes.
That said, humans are fallible and we've all made errors (except for me - I've always been perfect in every way. Just ask my wife, she'll tell ya' :). Good ones realize errors and fix them (ie. send payment ASAP). Good communication / simple FYI message can go a long ways in this arena.