I am not sure how long you should practice it. I think it is important to know what you have to do and you can combine the feelings exercice with inquiry as well. The feelings exercice can be used thoughout the day, whenever you have feelings ...
I use the feelings exercice and another tool (Sedona method) to let go of my feelings because I realized that my feelings hinder me a lot. I really love the feelings exercice for the power it gives me back.