Wait until a cell has a value and then do something

How while a userform is visible, I would like to query at regular intervals to see if a cell is no longer blank. When it has a value, I want to do something. Is there such a thing and would I put it in my form's initialize event? In VB I know there's a timer control that you can put on a form to do this, but I don't see it in excel.

Thanks!