I typically quarantine a minimum of 60-90 days. I was thwarted a bit on that here recently as spikes in daytime temps made my quarantine room too warm (86F ambient), forcing me to put everyone in my reptile room. Creative rearranging is keeping them on the other side of the room and no handling newer animals before handling any of my existing collection.
As for what I look for, first is mites. Not only can they weaken the snakes from blood loss, but they can also transmit diseases between animals. I keep an eye out for any signs of a respiratory illness, sluggish or abnormal behavior or movements, abnormal excrement (urates or feces), feeding habits, or anything else that doesn’t fit in the normal or expected range of behavior.