Most of my "solo" is normally the first leg of the travel to meet friends to then continue on, as most times I'm riding from the UK and they ride from Italy and we meet "in the middle".
I just had to cancel a trip to Spain over Easter, but the mechanic was the same, ride on my own to BCN over a couple of days, ride with them all the way to Gibraltar and around there, then split from the group mid Spain as they would go to BCN to take their ferry and I'd go to Bilbao to take my ferry.
So, about 4/5 days on my own in total. Mixing it with being with other people for the rest of the trip, makes it less "lonely".
A few years ago I did the Parpillon on the way south, and met another guy on top of the pass, while I was sitting by the tunnel entrance admiring the scenery.
We ended up riding the rest of the day together (he was very fast and very experienced offroad), then aperitif and we both went on our ways. It was nice.
I keep doing UK/Italy every once in a while on my own (3 days by bike) but it lost its appeal as I've done it many times and I started wanting to be more direct/faster in getting to destination.
But I always go visit the swans in this little town in the middle of nowhere in France on my way there or back. It became my little ritual.
I'm not 100% sure I'll be able to stomach a long trip completely on my own though. I know people that do/can do it, but not sure I'll love it over a certain amount of days.