I don't think there are absolute limits on the electromagnetic spectrum but I can come up with two limits: The longest wavelength for a photon would be on the order of the distance to the CMB radiation that started when the universe was only 379,000 years old. read more
In classical electromagnetism, there's no bounds on the frequency of waves: it goes from 0 Hz (= static field solution) to infinity. read more
There is no known principle limit on the range of possible frequencies of electromagnetic waves in free space. Of course, to practically produce very rapidly changing fields is not so simple. read more