I can assure you that there are few polling agencies out there who consistently use "time" as the basis of putting multiple polls into the field, even if they end up being non-random or event driven. If anything, the method I am employing here is more valid than what you propose because it attempts to capture the motivation/momentum after large moves in the market if they occur. Otherwise, it is exactly the design you propose with an interval, mine just happens to be 60 days instead of 30.