Agree with 45 minutes to an hour, but I find a more reliable indicator of doneness is inserting a knife into the center of the base (from the bottom, flip the 'choke over to get at the sweet spot). I've found leaves will pull off long before the bases are tender enough for me, and long before the heart/base of the 'choke is done.

Agreed 40-45 minutes is perfect!

Roughly 40 minutes or until you can easily pull out a leaf. Try not to over cook.

I usually steam them for about an hour, but start testing them after about half an hour or 45 minutes, depending on the size. The best way to test them is to use tongs to tug on one of the leaves - if it pops out without much resistance, the artichoke is done. Also, I don't know if you were planning on using a steamer, but I usually just put them in a pot with an inch or two of water, which works just fine. Enjoy!