What amount of maintenance is required to keep automated unit tests running/valid?

I am looking for a compare and/or contrast between the maintenance costs of gui automation and unit level automation. I understand the maintenance costs of GUI automation. But I have little experience with Unit level testing and the costs of keeping those assets running.

In the link :

Bjorn said the following:
"If you are treating your development correct, you should be able to re-use your tests when modifying or extending your software. "
<font size="2" face="Verdana, Arial, Helvetica">This would suggest that some maintenance is needed.

This link is a good overview, but is missing a gauge of the maintenance costs:

I am working on two auusmptions here:
Total hours = New Test Case Development + Test Execution + Test Maintenance
New Test Case development = 1/(Test Execution + Test Maintenance)

Is there a lot of maintenance in keeping automated unit scripts valid?