A realistic possibility across the whole season, and never something to count on. You might see one resting on ice, swimming near a colony, or patrolling the spot where penguins enter the water.
Late summer gives the best odds. February and March are usually strongest because inexperienced young penguins are taking to the water for the first time. Our own guides particularly rate late February and March for active feeding.
That is a tendency, not an appointment. Presence changes by site, by prey, by individual animal and by year. Some January voyages have superb encounters and some March voyages see nothing hunting at all.
On watching a hunt: it is possible near water entry points once fledglings start swimming, and you should decide in advance how you feel about it. It is not gentle, and some people find it upsetting to watch.
Nobody interrupts it, and no Zodiac driver should be crowding the seal or herding penguins toward it to make a better show.
On safety. Leopard seals are apex predators with a long head and a very large mouth, and their diet includes other seals. A resting one can look inert for hours and a hunting one moves fast and unpredictably. When guides call for a bigger distance than the usual minimum, that is why. Never get yourself between one and the water.