liquid back to a boil, lower the heat and simmer until tender, 10-15 minutes depending on their size. to test whether they are done, stick the tip of a knife into the base and the leaves, make sure both are tender.
one last note: to make sure the artichokes are done, i would taste one. you want to be able to easily bite through (and swallow the leaves). when i made this yesterday, the base was done but not the leaves so i ended up having to cut that part off and just use the base. 