Some leave about 8″ of yarn at the start and end of their project, but not in my house! In my house wool is precious and I think that's a waste so I just leave about 1” and I've never had anything fall apart yet.
Sometimes I darn as I go along but normally I do what I did last night - I'd crocheted 15 leaves, so I had a darning session with a pot of tea in front of the TV and got them all completed in one go. Yay!
Here is more info on how to darn in the ends with a needle...
If you want to leave a short tail
This is fine for little leaves as they are probably going to be sewn to a flower anyway, so are not going to unravel.
Take the yarn needle and weave it into your project so that the eye of the needle is near the tail you want to get rid of. Then thread the needle with the tail. Pull the needle through and taadaa, tail has vanished and is neatly tucked inside so that your project won’t unravel.