Do engineers have to keep going back to school because their knowledge eventually becomes obsolete?

If an engineer already has a degree I heard that their knowledge becomes obsolete after a while and they have to go back to school. If this is true, then how often do they have to go back to school and when they go back, how many units do they have to take? What % of what they learned when they first got their degree is still useful? I would ume that at least some basic things they learned is still useful.