There is no limit inside koala – the limit is the RAM in your phone.*
If you have a reasonably modern phone you should be comfortable with 30 mins to 1 hour of samples.
*Obviously the RAM on your phone must be shared with the operating system and other apps, so if you have 2GB of RAM in your phone, you might have much less for samples because Android OS will take maybe 500MB, other apps will also need some.
In koala, if you press the menu, it will show you how much RAM is available (see on the left hand side, “AVAILABLE RAM”) – 1MB can hold about 3 seconds of stereo audio in normal conditions but it depends on the sample rate and the hardware of your phone.