There's a lot of research that shows that working on two projects decreases productivity by about 40%. That means that in a 8 hour day, you get no more than about 5 hours of work done. It's a staggering 95% reduction in productivity if you work on 5 projects. That's about 30 minutes of productive work a day. The reason for the drop in productivity is switch time -- the time it takes to get into a task after you switch from another. For thinking work, like business analysis or programming, it takes 30 - 45 minutes to get "into the zone".
So, stay focused -- and work on one thing at a time for as long as you can. It'll get stuff done faster!