How long should we go on trying to falsify new scientific theories?

This question has its origin in recent discourse I had with a forum member I (already now) regard highly. I asked if after a new scientific has been proposed shouldnt we at some point just hold for true. Not for the time being only but always.

You can keep on falsifying and criticising but would this kill the theory? Most theories can be put to the test but what if the theory can be tested only theoretical, by means of other theories that can be tested in experiment. If the new theory contains elements that can't be tested in principle but that explain things that are testable what then. The Higgs mechanism is such an example but it's clear that that mechanism doesn't exist and is just a means to predict a real exising excitation of a composed scalar field.