flyingmad is spot on. Having been fortunate enough to date long- and short-haul Intl FA's (the female ones ), they "bid" for their flights - it is one of the most complex systems I have seen. They have to juggle a minimum flight hours requirement for the bid period with all the different flights, minimum downtimes, etc.

As an example, three years ago I flew on a "run" with my then girlfriend (with an overnight in each city, except SIN where I had 2). We flew MEL/SIN, SIN/PER, PER/HKG, HKG/SYD together. She also flew an overnight SIN/Mumbai return (straight turnaround). I was smart and stayed in SIN hotel, lazed by the pool, caught some rays

Seniority indeed plays a HUGE part...over 20 years and you are guaranteed to get what flights (runs) for which you bid.