I have pushed aggressively for a price reduction the very first week, when I knew it was in my sellers best interest. For example, if I do a broker open house the first week and receive feedback from the Realtor community that is consistent, I place great value on that and encourage my seller to take action accordingly.
How long will also depend upon the type of property and the tide at which the market is moving. I do not have a standard of 2 wks, reduce "x%". For example, historical homes frequently take longer to sell than newer homes. Non conforming homes that are unique for an area may take longer to sell. When providing pricing advice, I evaluate the comps, the potential pool of buyers for that type of property and the activity levels in the market. i.e. A home with 6 garages does not have universal appeal to all. It may require more time to find a buyer as a result. I guide my sellers to be more attractive than the competition. I have heard and read many "rules of thumb" over the years in real estate, such as this many days without a contract means "do this." I do find these rules of thumb to be menaingful for properties that are reflective of the stronger moving styles in normal markets. In a market not heavily saturated with inventory, where colonials rule, I would look for a price reduction by the second week, if we had no showings or if we had feedback to warrant such. In the advice that I provide for my seller clients, the "rules of thumb" are guidelines which I incorporate into the evaluation with many other factors.