Your question was essentially answered by Adam Savage and Jamie Heinemann from MythBusters. In short, combustion without oxygen will not work, therefore, for an explosion / ignition, a balance between the concentration of oxygen and methane / propane, etc. is needed. in room. The optimal level is somewhere in 10-15% of the gas, after 20% saturation occurs and there will be no explosion. Now calculate the volume in cubic meters of the room, and calculate 10-15% of it - and go ahead, farts. You see, in 10 years you will manage if the room is not ventilated, but by this time you have long ago discarded hooves either from suffocation or from boredom.
Somewhere, when, a very long time ago, I read that cows produce up to 500 cubic meters of methane per day (yes, the figure is staggering, but perhaps close to the truth), and yet with such indicators, half an hour of a cow's stay in the room, and hooray - BOOM!