Captains discretion and extended duty are two different things. The latter may be planned twice within seven consecutive days and is not subject to the captains approval (except if your CLA says something different), captains discretion may be used every day and is at the captains discretion after he has talked with every crew member about it. Extended duty has additional rest requirement, whereas discretion doesn't have any above the rest equal to duty time rule.
Extended duty is not in all cases 14 hours of course, if more than two landings are planned it reduces the same as any other duty period as well, for example it allows for a 13 hour day with four landings.
All the above applies to EU OPS subpart Q, as far as i know EASA OPS is in that regard very similar, but it comes into force next year, so we worry about that in half a year.