We investigate the coexistence performance of Wi-Fi and cellular systems under an unlicensed spectrum sharing environment. For this, we provide a mathematical framework based on queuing theory depicting the time-domain behaviors of a Wi-Fi access point (AP) and a cellular small-cell base station (SCBS) under unlicensed spectrum sharing. Based on the proposed framework, we make an analysis of the delay performance of both systems with respect to the changes in their packet arrival rates. Through the analysis, we identify the maximum allowable packet arrival rates of both systems, under which the required Wi-Fi delay performance is achieved without spectrum etiquette for coexistence at the cellular SCBS, such as carrier-sensing adaptive transmission. This will serve as a guideline for the cellular SCBS on when it needs to employ the spectrum etiquette.