I have had quiet a few in my lifetime and for a standard Japanese maple about 8-12 years if they are well taken care of. Even then there will only be a few here and there. There will be a few more each year. I get my seeds from two trees that were planted in early 1900. They have literaly thousands of seeds each year. I colected over ten thousand a couple of weeks ago. Japanese maples take a lot of patience!
Hope this helps
Kent