A: Pension benefits for World War I veterans evolved over the years. Basically the rule of thumb for a pension was having served for 90 days. See: Veteran's Administration guidelines.
The National Archives has set up a special site for requesting these records. The son or daughter of a WWI veteran should use that site to request his records.
As the granddaughter, you may access the records of retired veterans from World War I to the present at the National Personnel Records Center in St. Louis, MO.
You need to use Standard Form SF-180 Military Records Request Form to request those records. Click on the link to get a copy of that form. Simply complete the form and submit it to the appropriate mailing address listed on the form.