If you are a complete beginner, it may take 1.5 to 2 years before you feel like you are really sight reading. Here's a general rule: the difficulty of the music you are able to sight read well will always be about two levels below the difficulty of the repertoire you practice for recitals and such.
Is it hard to sight read piano?
With time, sight-reading becomes an easy process as the students are able to master the art and tricks involved in sight-reading. Here is some advice to all musical beginners: no-one is born a gifted sight-reader. This art, just like any other, requires a lot of practice and confidence in performance.
How quickly can you learn to sight read?
In my experience, it takes four to six months of piano lessons before students have mastered the necessary basics and sight reading can be successfully introduced.