Toxic friendships

I have found that periodically in my life, I have become trapped in friendships with women that over the years have grown toxic for one reason or another.  I don't see this happening as much with men.
Somehow women continue to meet and make new friends even after we're married.
My husband doesn't do that.  He came into our marriage with a set of friends from high school and college, and while he has met plenty of men since that he would consider acquaintances or softball buddies, he's never the one pushing us to have dinner with a new couple.  He finds an incredible amount of fulfillment through his job, our marriage and our children.
So why do most women continue to seek new relationships?  Is it that we grow, change and evolve more than our spouses, and therefore we outgrow some of our friends?  I think women are more emotional, we have a desire to talk, so much so that men cannot keep up with us- that's where our girlfriends come in.  We are always looking to find women with similar interests, children of the same ages, or some commonality that we can share in- this creates an immediate bond to build from.
How do you know your friendship is toxic?  According to Joan Baez...

• You don’t discuss your successes for fear of jealousy, criticism, or negative feedback
• You make all the effort to stay in touch; she never calls, she never writes
• You leave your visits feeling deflated, exhausted, angry, or depressed
• You’re constantly “cleaning up” after your friend
• You lie for your friend
• Your friend gossips or spreads rumors
• Your friend flirts with your boyfriend, husband, or life partner
• Your friend needs constant physical, spiritual, or emotional support
• You don’t respect or support how your friend is living her life
• You can’t be yourself in your friendship
• You dread taking her calls or spending time with her

I've often found that I give more or do more in certain relationships, but have recently come to realize that I don't take people up on their offers to help very often.  I don't like to seem vulnerable or needy.  I was brought up to think ahead- what would I want or need if I was in that situation, and to never take no for an answer.
I also grew up with a mother who did not have healthy relationships.  She had a very unhealthy marriage, and I don't think that she was a good role model in terms of letting someone be a good friend.
So the question is, when is the right time to walk away from a toxic relationship?  and how?