Where would the violence come from? Slovenia, Croatia, Bulgaria, Greece are in the EU. Albania and Montenegro have no territorial claims they are pursuing. Macedonia and Kosovo are limping along in a depressing manner but there's no violence there that I know of. Serbia is getting over the Kosovo affair. Bosnia is the only wildcard but I don't think even it can blow up again.

Of course, there are no guarantees in life, but if I were a betting man I'd say that for each extra peaceful year of peace in the Balkans you can add 1 year of peace in the future.

So my forecast is for at least 19 years of peace :)