When should I start decreasing my baby's naps?

Between 2/3 years . Usually between 2-3 years old but it depends on each child and your schedule. Sometime you will need the child to nap. There is no fast rule at all.
I think... ...The person to ask is the baby. Children act in a certain restless, cranky way when they actually need that nap; if they are consistently fine without it, you can skip it.