It is a pretty broad effect. It's why Democritus is the most useful hero during events in my opinion and in the opinions of many other players I have talked to about heroes. (Urephon/Deimos are probably joint second for buffing the myth units you get from events.)
Technically, I believe that it is city specific: Democritus must be in the city which you activate the token in, in order for you to have a chance of the token doubling. You can double the duration of a divine senses or heightened senses token, but you have to activate it in the city that you have Democritus in. If you activate it in a different city, then even though the token effects all cities, including the city that Democritus is in, it won't double. Democritus must be in the city which you have selected when you use the token.
(For the record, 48 hours of Divine Senses is so much fun.
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