This will mostly depend on how many miles you can walk in a day. Experienced hikers will comfortable walk 20 miles a day, while beginners may want to take a more leisurely pace at 10-15 miles per day. Give consideration to how many miles you've been able to walk in the past, and whether you'd like a relaxed trip, or more of a personal challenge. I personally wanted a challenge, and am used to walking 20 miles in a day, so I decided that hiking the South Downs Way in 5 days would be my goal. However, it's common for people to walk the South Downs Way in 7-10 days, so work out what's best for you personally.