Let [Formula: see text] and [Formula: see text] be two real numbers. For all [Formula: see text], the run-length function with respect to [Formula: see text], denoted by [Formula: see text], is defined as the maximal length of the prefix of the [Formula: see text]-expansion of [Formula: see text] amongst the first [Formula: see text] digits of the [Formula: see text]-expansion of [Formula: see text]. The level set [Formula: see text] is investigated in our paper. We obtain the Hausdorff dimension of [Formula: see text] which extends many known results on run-length function in [Formula: see text]-expansions.