Here's what I think. That we'll probably have much more than we need of people to do each job. That way each person just has to work less. This is a bit off from Marx's totally free system but I think that so much common labor should be required of everyone (something small like 2 hours a day or something, probably even less than that). That way we can get everything we need to be done to get done. Then after that people are free to work on whatever projects they might have.