At any time and as many times as the spirit moves you. But with complications. Some themes are similar, some have a different number of sidebars so you’ll have to work the widget selection again, a few don’t have widgets or have their own special widgets, etc. You must be careful with text widgets in particular, because with some themes you lose them, so it’s safer to copy their content and paste it in a simple text editor before you switch.
Oh – and you blog is http://stitchlady.wordpress.com – there is no @ in there. Take a look at the sticky, 8 Things, at the top of the forums for an explanation on how to link your user name to your blog.