I had heard that many European shops refuse to deal with them at all, but I cannot confirm this; I've heard the occasional horror story about AMEX taking as long as six MONTHS to pay a foreign merchant money that they should have had much sooner.
Over here the limit is 3 months (government laws) and it goes for ALL credit cards. Though, as apparently everywhere else, VISA payments go through quicker than, say, MasterCard or Amex. And actually, as of May 2012, "ordinary" consumers cannot get an Amex card over here, though they may apply for a Platinum card if they are eligible.
(Needless to say that I have a VISA debit card?
EDITED TO ADD: Of course, that also depends on which merchant bank is used. Most brick-and-mortar places uses DIBS/Teller (which is a Scandinavian corporation), whereas some online stores either use Pensio (based in Germany) or simply go with the Paypal option.