Summary: We consider the bounds on the non-commutative nature of space-time. We argue that these bounds are extremely model dependent. In the only phenomenologically viable framework, i.e. when the fields are taken to be in the enveloping algebra, the constraints are fairly loose and only of the order of a few TeV. We concentrate on the most stringent bounds that come from clock comparison experiments. In the framework where fields are taken in the enveloping algebra, they are model independent since these bounds are independent on choices involved with the Seiberg-Witten maps.