How long to feel a negative feeling

Hello! So I’m new here and still learning the model and the process. I’ve been observing my thoughts and when a negative thought comes up I immediately want to tell myself to stop and to change the thought. I know that we are being taught not to fear our feelings and to learn to experience them and not run from them. But how do we do this? How long are we supposed to “observe” the negative emotions and at what point should we find our way to a better thought? Some clarity on this would be most helpful! Thank you!