How many days do I need to incubate eggs?

It takes about 15 days to hatch eggs.The breeding period of this bird usually starts from April and will continue until July.Its nest is generally on the trees, and it usually uses tree holes or old nests. There are some soft things on the nest, and sometimes they do not put things.It can produce two to three eggs in one nest. The eggs are blue -green and there will be reddish spots on it.