According to a report by GamesIndustry.biz, Steam handles close to 50,000 refunds per day. As if May 4, 49,119 refunds were requested in a 24 hour period with these refunds being handled between 1.35 hours and 1.53 days.
Refund requests amount for more requests for Steam than all others combined. All other requests combine for a total of 16,535 with 12,366 of these being related to account security or recovery. 2,211 are filed under technical support, and 1,776 are attributed to purchase and billing support.