A flexible orthogonal frequency-division multiplexing optical network enables the bandwidth to be flexibly changed by changing the number of sub-carriers. We assume that users request to dynamically change the number of sub-carriers. Dynamic bandwidth changes allow the network resources to be used more efficiently but each change takes a significant amount of time to complete. Service centric resource allocation must be considered in terms of the waiting time needed to change the number of sub-carriers. If the user demands drastically increase such as just after a disaster, the waiting time due to a chain-change of bandwidth becomes excessive because disaster priority telephone services are time-critical. This paper proposes a Grouped-elastic spectrum allocation scheme to satisfy the tolerable waiting time of the service in an optical fiber link. Spectra are grouped to restrict a waiting time in the proposed scheme. In addition, the proposed scheme determines a bandwidth margin between neighbor spectra to spectra to prevent frequent reallocation by estimating real traffic behavior in each group. Numerical results show that the bandwidth requirements can be minimized while satisfying the waiting time constraints. Additionally measurement granularity and channel alignment are discussed.