For what it's worth:

On a Dahon Helios: about 35 miles
On a Bike Friday Tikit: about 75 miles
On a Bike Friday Silk: about 120 miles

On the Helios, constrained by the fit -- it's just not big enough to be comfortable for me on a long ride
On the Tikit, constrained by weight and stance -- it's a great commuter but for hills and wind I don't have it set up right
On the Silk, constrained by my fitness... I wouldn't hesitate to plan much longer rides if I were in better condition