Bitcoin is paid out in cycles of 2,100 Bitcoin blocks. During a cycle, all eligible stackers are expected to receive Bitcoin from miners. A cycle typically lasts for about 15 days, but can be shorter or longer depending on the Bitcoin block time.
Every reward cycle is preceded by a prepare phase of 100 Bitcoin blocks. This is when miners gather the Bitcoin addresses eligible for rewards in the coming cycle. To participate in a reward cycle, you’ll have to start stacking before the start of its prepare phase.
Every cycle 4,000 reward slots are available. The minimum amount required to get a reward slot depends on the total amoun of STX that's stacking and can change from cycle to cycle. If you don't meet the minimum, you can pool together with others.