It's flexible at this stage - I've spent 2200+ game days on my 1st and only world. No mods and not rushing to the 'next' boss or biome. I've done less than 1/2 of the listed challenges.
But that's me - many people crank through it to see how fast they can kill all the bosses. Others stay in creative mode to just build stuff, etc., etc.
I'm waiting to hear someone has started a yacht club for sailing addicts :-)