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The rule of thumb is about a month to recover from a long ride so 6 weeks is on the long side. But it does vary, and it increases with age.
Often people can do short stuff pretty quickly but it takes longer to do distance. Eg my 10 mile PB was set a few days after PBP,but I wouldn't have fancied a 25. I remember feeling pretty strong a month after Tcr in 2016. But my worst day on a bike was trying a 100 the week after a 12 hour.
frank9755-
Certainly does increase with age I've found, esp catching up on sleep. And in my case that's mainly longer tours/BP trips with long days but reasonable sleep rather than racing.
There was a guide or formula posted online somewhere from one of the original Divide racers, number of days spent at race intensity ie 18hrs+ on the bike and number of hours less sleep than normal to give time to recover. It was a number of months for a long duration race like the Divide. Probably also the generally sht food available there..
jameso
There are far more variables than just distance.
Not quite those ultra distances but I felt great after LEL (1400km) and carried on cycling with loads of enthusiasm, but PBP (1200km) two years later left me feeling fatigued and disinterested for months afterwards.
The main difference was fitness, I did the bare minimum to get to and then around PBP whilst my LEL year was a big year for cycling.