Beaucarnea stricta

Water this plant infrequently, when the top two inches or so of soil feel dry. Usually this will mean every week or two in dry weather. If you establish this pattern over several years, then you can cut back to watering every four to six weeks in dry weather. Use a slow stream of water so that it doesn't run off; allow the water to trickle all the way down through the deepest layers of soil. This plant will look more lush if given water every two to four weeks during dry weather once established. In a pot slowly water the entire surface until water comes out of the bottom of your pot.

If the crown of leaves gets smaller, and smaller, and smaller... you are probably under watering.