I suppose it's because armory needs to sync the blockchain... Altough Carlton Banks has a good point: if you're doing a fresh install, why not use the latest version?
https://www.reddit.com/r/BitcoinBeginners/comments/365wjm/how_long_should_it_take_to_sync_up_armory/A year ago, some people reported syncing durations of a couple of days to a full week. Since then, the blockchain has grown significantly, and the new version of bitcoin core is supposedly only a little bit faster.
In the end it's a combination of your internet speed, processor power, disk speed and memory... It can take hours to days... Impossible to predict