In Regency England, 1810's, a fast courier could travel from the Vatican to London in about a month. That is also assuming decent weather. A good horse, conditioned for travel can go about 40 miles in a 10 hour day when there are decent, fairly level roads or trails. You can push a horse faster, but only for a day or two then it needs to rest or you need another horse to keep going. The international cartographers guild has this guideline for horse travel: (miles/10 hour working day)Level or slightly rolling good roads: 40 Hilly: 30Mountains: 20Level or slightly rolling grasslands: 30Hilly grasslands: 20Level forest or thick scrubland: 20Hilly forest or thick scrub: 15Mountains bad trail: 10-15Marshland: 10Half those numbers if the horse has a cart or carriage, up to double if you push the horse really hard, or have a relay of horses.Vatican to Rome is about 1,160 miles. There were still Roman roads during the 600's but they were falling into disrepair, and they had to cross the Alps so I would guess it would take at least 47 days if the weather was perfect, the horse only had the rider and letter (and not lots of other burdens), and the rider knew where he was going and didn't get lost or attacked!