I can usually cram in 300+ miles getting the 07:30 out and the 19:30 home. Furthest I've been is across the Forth road bridge to Ladybank to visit Schtum, so it's possible to get quite a few miles covered. Here's a good wee site to give you some ideas of where to go. http://www.motorcyclescotland.com/
Coast road to Oban and round to Fort William, Spean Bridge, Newtonmore, back road to Aviemore, Nethybridge, Tomintoul, Braemar, Blairgowrie, home. That would be a long but cracking day, not sure of the mileage but a fair few