Is my grow-at-home mushroom kit ready to harvest?

Your goal is to harvest your mushrooms when the fruits are large but before they mature to the point of producing spores. As a general rule, you're better off harvesting a little early than a little late. IF YOU SEE DUST ON THE SURFACE AROUND THE BASE OF THE FRUITING BLOCK, IT IS RELEASING SPORES, SO DON'T WAIT ANY LONGER TO HARVEST!

For gilled mushrooms, like oyster mushrooms, the caps will generally flatten and eventually become funnel-shaped as they mature. You should pick them when you see them starting to noticeably flatten and before they turn up. Here are some pictures of oyster mushrooms ready to pick:

For toothed mushrooms, like lion's mane, bear's head, and coral tooth, teeth will develop and lengthen as the mushrooms mature. Once you see that teeth are noticeably developed, you can pick the mushrooms any time. The toothed mushrooms are quite a lot slower to grow and develop, as compared to oyster mushrooms, so there is more "wiggle room" in deciding when to harvest. The thumb rule that earlier is better, though, still applies. Here are some photos of toothed mushrooms ready to harvest: