Gacha session happens every 15 minutes for ten spins which means that every time you get a spin, you get one random card- if it happens to be one of the five ultimates -you win! :)
When the session ends, all players will get rewards depending on how many spins did they do during this session; keep in mind that everyone starts from zero, so even if you didn't manage to catch a single ultimate card during ten gacha sessions, don't lose hope because your next try might finally reward you with your ultra-rare creature! :)