Start when they are babies. Reading aloud in a soft voice helps calm and entertain even very young children. 20 minutes a day is not a magic figure, but it seems to be a good one. You may want to read more of course. Again, this is not a magic figure.
Some people wonder if it helps to read to children after they learn to read.
Experience seems to show that it does. Reading somewhat more difficult books than the child can read appears to help them increase their own reading skills. Make sure you give them a chance to read to you. Encourage the older kids to read to the younger ones. There are thousands of books available for every age group.