Many people are doing both reading and math at the same time.
Whether you do both and how long you do each for, as well as how long you show them for, largely depends on your child's interest levels and attention span, which will also be different at different ages.
I don't believe in hard rules. There is no such thing. At most, only suggested guidelines. As for me, I wouldn't hesitate showing couplets or even complete sentences from very early on. The good thing is that it gives the lesson VARIETY, which helps keep the interest.
I would also show some math lessons and gauge interest levels. If no interest, then maybe keep them till later, but you won't know unless you try!
