Note that within Star Trek, life support includes artificial gravity, inertial dampers, as well as HVAC. The Star Trek The Next Generation Technical Manual says (section 12.1) that
Other emergency provisions include distributed reserve life support systems, emergency support shelter areas, and contingency support modules intended to provide shipwide breathable atmosphere for up to thirty minutes in a major systemwide failure.
30 minutes is a very short period of time, although this is the ultimate reserve, which (in theory) you'd never need because all the other reserves would have sufficed.
The reserve atmospheric processors (which remove CO2 and replenish O2) were designed to provide up to 50% performance for up to 24 hours (depending on load). 24 hours, while longer than 30 minutes, still isn't a huge amount of time when you consider the distance from outposts an exploration vessel typically travelled. If using this system was necessary, then it would be important to act as efficiently as possible, to maximise the available time.
The Technical Manual has a lot of detail about how redundant and safe the life support systems are (all of sections 12.1 and 12.2 can be read using Amazon's "Look Inside" feature). Given that life support systems are this redundant, that means that if they are failing and the ultimate reserves are being used, then the situation is always very bad - so many problems must have already occurred, that the likelihood of critical failure is very high.
There's also an out-of-universe note about how this appeared to differ from reality:
Believability (not to mention crew safety) demands that the Enterprise environmental support systems be extremely reliable with many redundant backups. The problem from a television standpoint is this makes it a little tough to create story situations in which our crew can be threatened by life support failure. In one episode, "Brothers", writer-producer Rick Burman needed all bridge atmospheric support systems to fail. He rationalized it by having Geordi express amazement that seven independent safety interlocks had been bypassed, thereby acknowledging that the ship is indeed designed to make such failures extremely improbable.