Thoughts on practicing
Steve, this is good topic to bump.
When should you start to make something? When this details in your practice pieces resemble how you intended those details to look. When you feel, somewhat, in control of the situation. "Design Opportunities" and unrecoverable failures will still happen. Some of your projects will end up as firewood, but that's woodturning.
I'm fairly new to turning with occasional gaps due to work commitments.
I started turning a lot of 2x4s 12 to 18 inches long because they were available. Rip them square first. I didn't throw scrap wood away unless I turned it into shavings or made it round until it broke. Occasionally I'd turn some maple. What ever I could get my hands on-FOR FREE.
As I got more confident, I started making tops at demos, handles for my files with 3/4" water pipe for the ferrules, a shop trivet for my coffee cup from a 2x6 scrap, numerous lidded boxes and pencil pots. Although people would say, "that's nice" about a piece, I'd know the difference between what I wanted vs what I got. I knew I needed to practice more.
You're goal when practicing should obviously be to develop tool control. Set personal goals for yourself; try to turn series of even coves, beads, pummels, etc. Push yourself. Use whatever wood is available to you, preferably free wood. (Caution: avoid used wood because of nails, screws, metal, etc.) When you mess up, ask why and try it again, maybe on the same piece. Once you've learned to cut coves, beads, grooves, etc., don't be satisfied with something that's "pretty" until you can do it again. At a demo a well know wood turner said about developing tool control, when you make something, make five of them. She said then compare the first and last pieces. I apply that to practice pieces.
And never stop turning just for practice with 2x lumber and those odd pieces to hone your skills or learn new techniques.
Frank