How often do you have to drain water from a portable a/c?

If it incorporates a dehumidifier function, as many do, there will a tank to collect water. This usually has a float to stop the machine when the tank is full, and an indicator light to say it has done so.

a properly designed portable a/c will be set up so the condensate water will be evaporated and blown out the exhaust hose.

out the window or thru the wall types drain to the outside and likely need attention on this once a year at most.

no idea about other kinds -- never had one

The type that mount in a window have a built in drain and the water drips out to the ground when it gets to the level of the drain hole. Swamp coolers are not AC units but they should be cleaned when dirty. Basic reason to drain the water is when it gets full on the condensate pan. This happens sooner with high humidity than in a desert area.

When warm and humid air is cooled, water condenses from the air and must be drained from the portable a/c unit. Water needs to be drained more often from a warm and humid air cooler.

It is a swamp cooler? Most of the time you don't have to drain it unless you clean it, and they should be cleaned every 2 or 3 months (depending on how dusty it is in your area) BUT when you're not using it during the winter, you can drain it completely, but make sure you wipe it down really well or it WILL rust.