Seniority comes into it however there are more main crew "gaps" on longhaul than shorthaul generally. Most 737 flights have minimum crew so have one purser and two main crew. 777's have one cabin manager, one Purser (soon to be two) and 8 or 9 main crew. So even if you don't bid you should get 3 or 4 longhaul trips a month especially at the moment as so many crew are on high hours.