Finding feelings and for how long

When doing the model I usually have a hard time finding my feeling (always anger to start, seems this is where negative thoughts start). Do you have any suggestions on how to get to the feeling and determining if it is the right fit? Also how long do you sit with the feeling that is not self-indulgent and detrimental in staying with the feeling too long? How do you know this? When do you know you have been with your feeling long enough to move on? Thank you!