Typically we indicate the reed contacts open within 50 microseconds. However, this does not mean that you can immediately apply a voltage across, particularly if you are working with high voltages. Breakdown voltage or dielectric strength is stipulated for a given AT range (mT range). For high voltages we use the highest range that will work for a given set of pull-in voltage requirements. With that said, when the contacts first open they will waiver for at least a few milliseconds. During this time the contacts will come close to closing in a periodic manner. During these close encounters the dielectric strength will be much reduced. If this is a consideration in a customer circuit, care must be taken to wait a sufficient amount of time before re-applying the high voltage.