In a similar context, Samael Aun Weor states that it is necessary to endure a practice daily for at least one year, without skipping a day and patiently going through all the set backs and the discouragements. So naturally, out personal assumption would be to approach this Clairvoyance exercise in the same manner.
Please be aware, that there are additional factors, like your mental habits, ethics, the kind of impressions you take in, your karma, etc. So this can be very individual and might even take years of trial and error.