How long is too long in a relationship to say love?
Totally situation specific. For me, it's usually around three months that I start considering it, and around six that I feel ready to say it. Very loosely.
Who brings up exclusivity and if no one does - both are understood to be dating other people?
Either of you can bring it up. Sometimes nosy friends or relatives bring it up.
But yes, I always assume unless the conversation has taken place, there is the distinct possibility that either person is seeing someone else. Personally, I end other things before even going into that conversation. In my experiences, the men I've dated have done the same, and we've already been on the same page by the time we have the conversation, but then, we've also danced around it before it "formally" came up.
How long before exclusivity?
Totally depends. Usually a month or so for me. I'm also of the mindset that I won't be intimate with someone until we're exclusive, so that can influence timing sometimes.
Interesting my friend and I had very different views. She thought that exclusivity could only be brought up by a man.
I don't agree with your friend, but if she's uncomfortable bringing it up, or is really into being pursued, she's certainly not obligated to bring it up in her relationships.
On the other hand, she's not really entitled to tell you how to run your relationships either.