I really don't think there's a right or wrong amount. I do what ever I have to to make it sound like a real animal in distress. Chances are there not going to go for 5 min. Straight or at the same volume. I make it sound like its being attacked by playing with the volume. Make realistic and believable.
Make it sound realistic. What would a rabbit sound like if it was being attacked or stuck in a trap? Wounded by a car? One reason I sometimes use a mouth call is to get emotion involved. Again - think of what a dying rabbit or other critter would sound like. I like to think at first it is going to go ballistic, then as your sequences progress, the animal is going to get weaker and weaker. Think about it.
A gray fox or even a red fox isn't THAT much bigger than a rabbit. In my opinion, you want to make that fox think the rabbit isn't going to kick his / her guts out, when coming in to finish off the "deal".