Research has shown that single stuck-at fault (SSF) N-detect test sets are effective for detecting defects not modeled by the SSF model. Experimental results showed N-detect coverage is a good metric for determining test quality. In this paper, we examine the test quality of pseudo-random Built-In-Self-Test (BIST) patterns by quantifying the relations between their N-detect coverage and test length. We theoretically derive bounds on the minimum test length of pseudo-random patterns required to achieve a given N-detect coverage. For faults with high detectability, the expected test length for N-detection is around N times the expected test length for single detection. However, for faults with low detectability, the expected test length for N-detection can be NlogN times the expected test length for detecting the fault only once; this increases the test length significantly. We also introduce the idea of effective detectability which is important for analyzing the effectiveness of BIST techniques for detecting real defects.