Developer’s productivity can also degrade by over 50% for each additional simultaneous project. […] When developers must contribute to multiple projects, make sure that they are guaranteed at least two full days on each project before switching to another. This will minimize the amount of time they must spend reintroducing themselves to each project.
James Leight, Provide Regular Time to Focus. 97 Things Every Project Manager Should Know. O’Reilly. 2009