If they come in too fast, the gravity wells get out of sync. Owners of gravity planes carry a larger amount of personal atmosphere with them if they enter wildspace under their own power. Large creatures and above have a variable envelope depending on their length in feet. The length to be used is the longest measurement of their body. This is a guideline, however, and you may have to adjudicate some odd creatures that are very long but thin, or very round, etc.
Air quality goes from foul to deadly in the same amount of time it took to get to foul.
That's what the Air Numbers are for. Let's use Nelson the red dragon as an example. He is 45 feet long, so he has enough air to last himself 90 days. Add up the Air Numbers for each passenger and Nelson. The total for all the passengers and Nelson is 37.