There is a lot of debate still on the question of whether a virus is alive or not (eg here). If we can't decide on something so familiar to us, I would certainly be hesitant of making declarations about alien forms. To take the time-based example, a creature with very low "metabolic rate" (or its equivalent) and a lifetime of 1000 years would be very difficult to detect simply because our attention span might easily miss any significant happening if we only watch it for a year or so.