I am screening soybean genotypes for drought tolerance and vulnerability in a screen house. Some literature suggests 3 weeks and others suggest 4 weeks of drought imposition.
Dear Sir make an experiment with 0 1 2 and 4 week drought imposition. The curves you get should give you an exact protocol which will work for your conditions and this will be regardless of what other literature suggests.
Your own information will be bestfor your own conditions.
As Paul Reed Hepperly said, your own data is the best. Also conductance measurement and mean time of evapotranspiration are good measures to have an idea of drought mean point.