You should connect to your drawing area's expose-event
signal. That is the only place that you should try to draw on the drawing area; the reason for this is that anything you draw is erased again when the window is minimized or another window moves over it. However, the expose event always happens at the right time so you can keep the drawing up-to-date whenever it is needed.
Like this:
def on_drawing_area_expose(drawing_area, event, data=None):
# ... do your drawing here ...
drawing_area.connect('expose-event', on_drawing_area_expose)
Also check out drawing with Cairo, which is the preferred and more flexible way. Here is a tutorial.