How often do you water a yucca tree?

Mine is looking bad. The bottom leaves are drying out and brown. I've watered it twice in the two months I've had it. I've trimmed them and put a growing light behind it. I don't know if I've over or under watered it. I water it until the water pours out the drain holes and I put pebbles on the bottom of the pot. I've tried to do everything right because I love it. Is there anything I'm missing?