This will depend on the type of sample being analysed. Mushroom and yeast samples generally contain < 10% α-glucan. However, some commercial mushroom mycelia are grown on cereal grains, and in this case, the starch content of the recovered product can be as high as 75% w/w.
If in doubt, you could run on the assumption that the value is 0-10%, then if the value is very high (absorbance values for the sample are higher than the absorbance values obtained for the glucose standard), dilute an aliquot of the solution 10-fold and repeat the GOPOD reaction.
The absorbance received for the sample must not exceed the absorbance values obtained for 100 μg of glucose standard (0.1 mL of standard at 1.0 mg/mL that is supplied with the kit).