Like I said, I'm not sure, but they do sometimes carry decompression gas or gas for emergencies.
I am telling you that they can go at any length as long as it is habitable for them. Sometimes they  include additional cylinders for decompression gas or emergency breathing gas.
Scuba Diving is an interesting thing.  There's a lot of ocean science involved. Yes, scuba diving involves being completely untethered from the surface air supply. Have you ever been scuba diving? No, I haven't. How far down can scuba divers go? I don't know for sure, but divers do carry their own breathing source, and therefore have more freedom to dive. If they use gas as the form of breathing, at what depths can scuba divers descend?
They may include additional cylinders for decompression gas or emergency breathing gas.