Hi Mikey, it's my understanding that when the queen runs out of room to lay eggs, is when they decide it's time to swarm. In a 10 frame hive I would say that if 8 of those frames are full of brood, larvae, eggs, honey/nectar, pollen... that is a good indication that a swarm is coming soon. So if you have two 10 frame hive bodies = 20 frames, and it only has 5-6 frames full, then you have some time to buy. But keep a very close eye on this hive, like check it once a week, the bees can fill up that hive quickly.