yes..different systems for different A/C..i worked and trained...

yes..different systems for different A/C..i worked and trained in emergencys for 20 yrs. for example on some 767 chemical generators last 12 min..some 15 and A330 22 min..by memory (Dont quote me on the exact timings)
Depending on A/C. Not all are the same and i guess it
changes when new technologies come available..making the units lighter and smaller as time passes by..its all about weight in the big picture of schemes and construction..the lighter the units the lighter the plane the bigger the payload. In reality..all you need is about 12 min of o2..plenty of time to descent to 10000ft..after that o2 is no longer required..If you cant be at safe altitude by then all the o2 in the world wont save you!(can t recall were exactly) but if flying over some high terrain there is a requirement of 20 min o2..(before one is able to decend to safe altitude) and i guess maybe thats why the 74's have the bottle system as they are long range A/C and fly continent to continent..but also could be something that company's choose as a fit out when ordering..again costs, maintanence, A/C usage and weight are taken into consideration.