Q: To obtain venous blood gas (VBG), what is the minimum time for the tourniquet to be off? 

Answer:  At least a minute

 If a tourniquet is used to facilitate venipuncture, it should be released about one minute before the sample for venous blood gas is drawn to avoid false values in VBG induced by local ischemia.

Reference:

Cengiz M, Ulker P, Meiselman HJ, Baskurt OK. Influence of tourniquet application on venous blood sampling for serum chemistry, hematological parameters, leukocyte activation and erythrocyte mechanical properties. Clin Chem Lab Med 2009; 47:769.