The ability to enable this prompt can be controlled within 'My Settings.'
Dependent on your process, you may or may not want to enable this window prompt for each of your users.
Please refer to the steps below for how to enable/disable this window prompt:
1) Login as the respective user
2) Select 'Tools'
3) Select 'My Settings'
4) Highlight the 'Estimate' tab
5) Enable the checkbox entitled 'Use Quick Quantity on Estimate'