How long should the delay be before a tooltip pops up?

I'm working on web site in which a "ribbon" submenu pops up as soon as the mouse touches one of the main menu items. I'm trying to explain to the designer that it should wait until the cursor has rested on the menu item, and not pop up when the mouse just happens to pass over it. I'm not sure how long (in milliseconds) that wait should be.

I can't link to the web site in question, but I figure the same principle applies to standard tool tips. How long should it take for a tool tip to pop up?