How long does it take to ride from Scotland to London by bike?

Depends on how hard you want to ride, and where you consider Scotland to start. By the road it's about 700 km between London and Edinburgh. A really fit rider would be able to cover that in two days riding if he's travelling light. Anyone who can take 4-6 hours in the saddle daily should be able to do it in seven days, even when carrying gear. For someone with a bit of experience in bike touring I reckon 4-5 days would be a nice target.