You're only allowed to take batteries onto an airplane that are less than 100 watt-hours (7-ish amp-hours for a 12 volt battery).
If you are OK with "transportable" instead of "portable", it's tough to compete with a 35 amp-hour deep cycle AGM battery intended for electric wheelchairs and scooters. I paid $63.00 for mine with free shipping and no sales tax. It weighs 25 pounds so you're not going to toss it in your backpack and do the Grand Canyon rim-to-rim hike but you can use it car camping, on the boat, and in your bedroom during a blackout.
Battery life is variable depending on your machine settings. Somebody with low pressure settings who can get by without a humidifier or a heated hose isn't going to need much power. A 95 watt-hour lithium battery legal on an airplane would last a night. The best thing to do is put your machine on a power meter and see what you use. See if you can live with the hose and humidifier heaters turned off. Everybody is going to have their own answer for how much power they need.