Is It ok to store your bike outside for 8 hours/day? The place where I work has some bike racks outside with some basic roof on and not much else. That means if it rains it won't fall directly on the bike, but otherwise it is exposed to the elements. Is it ok to store my expensive mountain bike there every day, all year round, for 8 hours/day? Or will it get damaged over time?
If weather is the concern, yes.  You will get some damage over time.  Most of that will just be accelerated wear of all the normally wearable components.  Keep your chain lubed or it will rust.  You will go through grips, tires and seats faster.  Any bolts that aren't alloy or stainless will rust.  Non-stainless cables will rust through.  

That is not the only concern though.  You will be exposing your bike to a greater opportunity for theft or vandalism, especially when it becomes obvious that bike sits there all day.