I've tested my single infusion mash at 30 minutes before and showed full conversion of Briess 2-row. Like Yuri says, in most of today's modern malts, there's way more enzymatic activity right out the gate than what our forefather brewers experienced.
I typically never mash more than 45 minutes before I vorlauf and start the batch sparge.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10