It depends on distance. 1 mile, 6:30 min/mi, half marathon 7:40 min/mi.

There's quite a good formula that Pete Riegel came up with to predict your speed over a certain distance: T2 = T1 x (D2/D1)^1.06. There's a caculator on this page: http://www.runnersworld.co.uk/news/article.asp?UAN=1681. It's been pretty accurate for me, YMMV.

7:20-7:50 minute/mile seems to be the equilibrum of fun/ease. Under 7 minutes is pretty painful, over 8 feels pretty slow.