I'm currently working as an intern in a Web company and its the first time that I have to work 8 hours a day (I'm in college so that's completely new to me).
How do you guys stay focused for 8 hours? It's kinda hard to go from 90 minutes classes to 8 hours of (somewhat) consistent work.

How do I focus for a long period of time?

Take breaks

Learn to get into the "zone".

Also learn how to get out.

I think the point is that you don't have to concentrate 100% 8 hours long.
For me it's important that I take breaks and do things from time to time which are easy to do like sorting emails or cleaning up my desktop (virtual and physical) to have everything right in place when deep diving into the next ticket.

For me its all about interest. If I find something interesting or challenging and get into the zone I can forget to eat for the whole day ;)

But if I cannot find any interesting, or if I am interrupted to much I do have the same problem of keeping focused and need frequent breaks to gather my thoughts or get a new view on the problem.

The secret is that no one works for 8 hours straight, that's non-sense. Everyone takes small brakes all the time. Coffe breaks, watch YouTube, smoke breaks, chat with colleagues, scroll 9gag ... Development is a pretty mentally demanding work so mental hygene is important in general.

We don't. Nobody does as far as I know.

I simply focus on tasks for the day not the number of hours I put in.

Usually accomplish 50% of the daily tasks while trying to finish 100% of the tasks and chill for the rest of the time.

@Hazarth and we should not forget than when problem solving, the brain often keeps going in the background, matching up memories and such.

So after the break we might suddenly have the solution popping up to to conscious level :)