Advance-reservation is an essential feature of any system in which resources may need to be co-allocated at predetermined times. In this paper, we discuss unconstrained advance reservations, which use flexible time-windows to lower blocking probability and, consequently, increase resource utilization. We claim and show using simulations that the minimum window size, which theoretically brings the blocking probability to zero, in a first-come-first-served advance reservation model without time-slots, equals the waiting time in a queue-based on-demand model. We also show, with simulations, the effect of the window size on the blocking probability and on the resource utilization, for an advance reservation model with time-slots, for different types of arrival and service times. We then compare the blocking probabilities obtained by on-demand reservations, advance reservations, and unconstrained advance reservations with flexibility.