This often comes up...once more into the breach. From the Brooks website

" The primary function of Brooks Proofide is to nourish the leather to keep it supple and to prevent it from drying out with age. With this in mind, it is generally not necessary to apply Proofide to a new saddle. Instead, it is recommended to apply it after every 6 months.
Proofide should be applied sparingly to the upper surface of the leather using just enough to cover the entire upper surface area. This should then be left on the leather for several hours (perhaps overnight) in order to allow penetration through into the inner fibres, before buffing off thoroughly with a soft cloth."

So that's what I do.