So, about three in-game weeks ago my party hatched an unknown egg that have been carrying for quite a while, which turned out to be a thunderbird (it was originally a Roc, but due to an incident the egg had obscene amounts of magical energy pouring through it for several weeks).

So, how long should it take a Roc/Thunderbird to grow to adulthood? They are going to be raising it, and are currently trying to figure out how quickly they need to have modifications made to their abode to hold it. I've been reading around and I can' find any definite answers anywhere.

Edit: System we are playing in is Pathfinder. The creature description is here