Depends where abouts in europe. Usually it is just until your return date, but some countries insist on an extra 6 months afterwards. Check with the embassy of the country
Also depends on your nationality - some European countries just want a valid passport from a UK passport holder but may want 3 or 6 months from an American or other 'foreigner'.
Finding this out for sure can be tricky - some Embassy websites are wonderfully unspecific on this subject.
you have to renew your pass port when you have 6 months left on it it states this on the renewal form. its a con really because they are doing you out of 6 months.
just b carefull because no matter where you travel if you have less than 6 months left customs can refuse to let you travel.
The UK Passport Office website says that when you renew your passport they will give you credit for any unexpired whole months - minimum one month, maximum nine months. So you won't lose out if you renew before your existing passport expires.