There isn't really a "forumla" you can use to make decisions based on limpers, but consider this before you decide :

1. What are the blinds in comparison to my stack?
2. Is the table playing generally loose or tight?

I would love to help you more but your question is a little too general for an answer. If you would post some hand histories from situations where you were weren't sure of what to do, I'm sure someone will give you some good advice. Also, is this question for ring games or tournaments?