At least a few days a week. Gives people a better understanding of the big picture of what everyone is working on. I know collaboration is the hot button word and over used but there is truth to the fact that it's important and can't be done as well remotely.

Now there are certain some jobs were people don't need to see the big picture and don't need to collaborate at all.

It also depends on the employee for every employee who "just at productive at home" there are probably 5 that are not.

Also easier for managers to manage in person and keep track of what it being done. This can be overcome with better management but most managers suck.