The LICOR 6400 protocol suggests using a minimum wait time of 120s with a maximum of 200s. While stomatal conductance will be correctly calculated in this situation, it is not the steady state value that will be obtained. If the light or CO2 or leaf to air vapour pressure difference is changed, it takes stomata about 20 minutes to fully respond due to the hydraulics of the system. An example using light flecks provides a vivid illustration of how the signal and response time can get out of kilter: Kirschbaum M.U.F., Gross L.J. & Pearcy R.W. (1988) Observed and modelled stomatal responses to dynamic light environments in the shade plant Alocasia macrorrhiza. Plant Cell and Environment 11, 111-121. The time required to reach steady state also differs between species and particularly between dicots and monocots.