Between 20-30% would be my best guess. I'm including business emergencies as well, regarding client support. These emergencies get me out of the groove, which also adds up wasted time, while I try to get back into what I was working on.
If I can put it in reasonably defensible numeric terms for the managers/directors who can purchase software ("I can save 5% of the time of your senior/lead engineers!"), it'll be much easier to sell.