How long for new weapons?
I would assume it’s for a limited time like other signature NPC weapons. I would assume two weeks but it may be as little as until the next Tuesday update.
RIP City of Heroes
It’s rare that they’ve introduced gem shop items for less than two to four weeks without telling us: they tend to want to encourage a “hurry up and spend” attitude, where possible.
Based on recent practice, you will see a cooldown timer on the sale items at least 48 hrs before the end of the availability. Sometimes (but not “often”), they’ll announce a change in the gem shop offerings via the main guildwars2.com page.
I speculate that B’s Greatsword will be available at least through the first two weeks of the feature pack and probably until the next Living Story update. (But I’ve been wrong before.)