If it's criticism like "you suck" or "your artwork sucks," we would argue that there is NO right time for that (unless the person is into it, we guess).

If it's criticism like "I like this about your artwork, but this part of your artwork could be improved," the safest things would be to wait until the person asks for criticism or indicates that constructive feedback in allowed at all times. Unprompted criticism can feel like you're saying, "I'm choosing to find fault with you and your creativity," even if that's not your intention.

You don't have to be an expert in whatever you're critiquing to offer valid criticism. But when in doubt, ask with the individual if criticism is okay.