Based on pilot study research from about 400 individuals in one study and about 1000 in another, the response rates for the DHQ I varied from 70-85%. In both these studies, the DHQ I response rates were not statistically different than those from shorter FFQs. Based on the first study listed below, the questionnaire takes about one hour to complete. The above findings likely apply to the DHQ II, but to date, no such studies have been conducted or are planned.
Subar AF, Ziegler RG, Thompson FE, Johnson CC, Weissfeld JL, Reding D, Kavounis KH, Hayes RB; Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial Investigators. Is shorter always better? Relative importance of questionnaire length and cognitive ease on response rates and data quality for two dietary questionnaires. Am J Epidemiol. 2001 Feb 15;153(4):404-9.
Subar AF, Thompson FE, Kipnis V, Midthune D, Hurwitz P, McNutt S, McIntosh A, Rosenfeld S. Comparative validation of the Block, Willett, and National Cancer Institute food frequency questionnaires: the Eating at America's Table Study. Am J Epidemiol 2001 Dec 15;154(12):1089-99.