As much as possible! Yes, a toddler's need to be active coupled with your adult life and obligations will make it impossible to read for hours on end. But you really can't read to a toddler too much. When busy schedules make it hard to fit in reading together, try some special, designated reading time before naps or at bedtime.
You can also encourage your toddler to practice reading to you. This is especially true if you're reading their favorite book, as they'll already remember much of the plot. This will help them practice using vocabulary and creating stories.