Valid 1 year after the first flight (which needs to be less than 1 year after you buy the ticket). So the max. validity could be almost 2 years after purchase.

You can either book certain flights at certain days (not possible if you buy the ticket more than around 330 days before the flights).
You can also leave the ticket open. Advantage of open is: nobody not knowing the rules at checkin will hassle you when you check in at a (otherwise) different day. Disadvantage: If nobody flies that sector any more you need to reroute the ticket and pay. Otherwise the change is free.