Once I've found a worldgen I like and intend to play in, I find I tend to burn out on it in approximately a month or so. Never been able to pinpoint why precisely, but I suspect it's because that's roughly how long it takes for me to get whatever project has engaged my interest done. Once that project has been finished, I find it hard to re-interest myself in whatever the world has to offer and there's usually some shiny new mod I'm interested in trying out.