it's about 8hrs I think (longer if you 100% it). There's another different little mission that unlocks after completing the game where you play as a different character.
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That sounds a perfect length! With this sort of game I tend to go for the 100% too. And if the demo is anything to go by I'll enjoy relaying it plenty.
With the Demo being essentially a prequel do youthink that could also be added to the main rom as an bonus menu option one day? It could be renamed 'Goodboy Galaxy Origins' or something?