ViaMichelin's recommended route from London to Pisa gives the driving distance as 1484km (= 922 miles). That includes 1401km (= 871 miles) on (boring) motorways and just 121km (= 75 miles) on designated scenic roads.
The total driving time is given as 15 hours 19 minutes (including 12 hours 33 minutes on motorways). However, that total only includes 1 hour 16 minutes for the ferry. Allowing for checking in, you need to add at least 2 hours to that total time.
So, with just one person doing the driving, it could be done in 2 days if the driver simply wanted to get there as quickly as possible (and not do any sight-seeing en route).
Sticking mainly to the motorway route, but with some detours to explore local towns, 4 days might be more sensible.
Switching ViaMichelin's preferences from 'recommended route' to 'discovery route' changes the total driving distance to 1352km (= 840 miles), with only 380km (= 236 miles) on motorways and 352km (= 219 miles) on scenic roads. Taking the slower roads increases the total driving time to 24 hours 42 minutes (with 3 hours 38 minutes on motorways). Once again, add a few extra hours for the ferry.
Taking the 'discovery route' would thus require a minimum of 3 days driving but that would seem to negate the point of 'discovery'. (i.e. there'd be no time for sightseeing). 5 or 6 days would be more sensible.
http://www.viamichelin.com/
Chris