I don't have any experience with Japanese maples but common sense would say to wait until the maple itself decides its seeds are ripe. i.e. when they start to fall naturally. I have grown other trees from seed which I have just collected up off the ground. Why make things harder?

When the 'wings' on the seed begin to turn tan, but before they have become completely brown and woody, is ideal time for collecting the seeds of acer palmatum/Japanese maple. That's late summer into early Fall here.