The glossary has come great advice on Curses but doesn't explain how to rate them as challenges. Same's true for most hazards. The rules might give a CR for specific dangers (like green slime, for example) but, far as I can find, doesn't give a methodology for rating your own home brew threats that aren't traps or haunts.
Thinking I'll apply the same philosophy as the Haunt rules, but not all curses are based on Bestow Curse or other spell effect. For example, how would you go about assigning a CR to the Curse of the Ages?