A tale as old as time: early intervention has lower costs and higher probability of eradication success. ”Early” has been long engrained in invasion biology theory, but what constitutes ”early” remains vaguely defined. We argue that a critical transition between early and late intervention occurs when the spatial extent of the invasion becomes sufficiently large that we can no longer apply management effort to all parts of the population, and a more complex management strategy is required for successful eradication. The ”critical area” proposed here is defined by three factors: growth rate of the species; budget available for the management campaign; and effectiveness of surveillance and control technology. We derive a simple formula for the critical area and explore its effectiveness as a heuristic, using simulated invasions. Results from simulations in 2-Dimension show that starting intervention before the critical point has a 100% success; starting intervention at the critical point has a 78% success rate; and starting intervention after the critical point, has less than 1% success rate. We also examine how this heuristic might be used to guide decision making around the quantum of investment and optimal strategy to achieve eradication.