Motorcycle - from Dover to Rome - how long ??

A good friend of mine is motorcycling next week from the port of Dover to Rome. He was asking how long did I reckon it will take? Anyone on this forum completed this trip......if so, how long did it take? Or, does anyone have any idea how long it will take?
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a lot depends on the comfort offered by the bike - a tourer like a pan-euro and the likes will make 500+ miles a day achieveble - but my sportsbike (which needs refuelled every 150 or so miles) would be torture at or around the 350-400 mile mark.
would be a cool trip though - i'd try and get an early morning sailing to calais, stop the first day around the north-west france german border - head through switzerland the second day and aim to stop in Italy around Milan / lakes region and then press to rome on the third day.
actually thinking about it - i'd have to go for a 4 day trip for this - otherwise i'd just be in agony for days afterwards too.