Function minimization as genetic algorithm stop condition

I have implemented genetic algorithm for a problem where I have an objective function which I need to minimize:

Cost = ax + by + cz -> min 

The genetic algorithm should have found the optimal solution if objective function is satisfied.

The function will never reach 0 or below, so I just need to minimize it. Now, since there are no objective values to be reached, how do I know whether the objective function is satisfied or not?