My first solo trip was just 1 full day, a Mother's Day gift from my family. I flew there Friday night, did the parks Saturday and flew home on Sunday. The only good part was that this was back in the day of the non-expiring passes, and for tickets, I used the last day of a 6-day park hopper we'd had from a previous trip, so even for just one day, I DID park-hop. Since then I've done all different length trips, lots of 5 day trips, longest was 12 days. I find that about 8 days seems "best" probably", but there's no "bad" trip length for a solo trip!