Answer: If there is any way for you to punch holes in the bottom of the plastic pot, I'd strongly urge you to do so. While the two inches of rocks in the bottom of the pot will be a place for excess water to collect, there's no where for it to go and it can become sour very quickly. Warm weather will encourage bacterial growth. Aside from being odorous, the tainted water can mix with the fresh water you use to water your tree and it can flood up into the soil and affect the roots of your tree.
Once you've corrected the lack of drainage problem, you can water your tree at will to keep the soil moist but not soggy wet. Frequency of watering really depends upon the weather and how much new growth your tree is putting out. I would allow the top inch of soil to dry, then water thoroughly - until water flows freely from the drainage holes. Once in a while you'll want to either immerse the container in a larger container of water, or flood the soil by drenching it two or three times in succession. The idea is to completely saturate the soil to force out any air pockets that might have developed within the root mass. This happens quite often with containerized plants so flooding the soil will help eliminate the air pockets.
As for feeding, since your tree is in a container it will be safest to feed lightly but more frequently. I'd use a water soluable fertilizer such as Shultz's or Miracle-Gro, diluted to half-strength and applied every 2-3 weeks during the growing season. this method of feeding supplies a constant source of nutrients without the concern of over-feeding containerized plants.
Best wishes with your orange tree!
Q&A Library Searching Tips