In the footer of the subform, add a textbox. In the ControlSource for the textbox enter the following (without the quotation marks) "=Sum([name of your hours field])" (don't forget to use both the round and square brackets). Then, on the main form, add a textbox where you want the total hours shown and add the following to it's ControlSource (again, without the quotation marks) "=name of subform.Form!name of textbox on subform". For example, if the new textbox on the subform with the total is called txtTotal and the subform is called Projects, the ControlSource for the new textbox on your main form would read as follows "=Projects.form!txtTotal" If either the name of the subform or the total textbox has spaces (i.e., subform name = Project Details) then make certain that the name is enclosed in square quotes (i.e., [Project Details])
Hope that helps.