Wait until the seeds have matured/ripened on the tree. This happens from Summer through Fall, depending upon your location. Some indicators that the seed is ripe will be seed-cases that are full-size; seed-cases that have begun to turn from green to brown; and evidence that other seed-cases have begun to fall from the tree.

Many people prefer to harvest seed before it is fully dried on the tree. I take handfuls of seed and throw them into a small cardboard box to finish drying. I don't leave them in direct sun.

Now, the final caveat:
I've never grown maple from seed before; but I have collected seedlings from seed sown outdoors the previous Autumn. Next Spring, I'll give my collected seed a try.

Josh