'Add to Cart' change globally to 'Quantity'

Hi, My client wants to change the verbiage 'Add to Cart' to 'Quantity'.
not the button name itself, but the text that is displayed next to the Quantity input field.

where can be this be done? i tried looking in that developers toolkit page and found the constant for the button which i will try but i don't that's it,

thanks

When I searched for add to cart in the Developer's Tool Kit, it brought up /includes/languages/english.php:

Line #366 : define('PRODUCTS_ORDER_QTY_TEXT','Add to Cart: '); 

fantastic, thanks!

And of course you should make the change in a copy of that file in /includes/languages/your_template/english.php, where all customized sitewide text should be.