Monitoring trails (m-trails) have been extensively studied as an alternative to the conventional link-based monitoring approach by using multi-hop supervisory lightpaths in all-optical networks. However, none of the previous studies have investigated the effect of length constraints upon the m-trail formation, which nonetheless correspond to the failure localization time. This paper addresses the above issue and formulates a new m-trail allocation problem, where the relationship between the number of m-trails versus the maximum hop count is explored. First, the paper investigates the theoretical bounds of allocating m-trails with at most k hops via an optimal group testing construction. Secondly, a novel meta-heuristic approach based on bacterial evolutionary algorithm for solving the length-constrained m-trail allocation problem is introduced. Through extensive simulations the performance gap of the proposed algorithm to the lower bound is presented on a wide diversity of topologies.