How long or how many dates should you have gone on with someone before you decide you want to be serious and date this person exclusivley.

This is a multifacited question. It depends on the age, maturity and education of the couple. By education, I mean about safety, morality, respect and responsibility. After that, it's basically everyone's choice to make on their own.

How long or how many dates should you have gone on with someone before you decide you want to be serious and date this person exclusivley.

No such magic number. Let it happen naturally, eh?
:)

TAKE YOUR TIME!! The first two months you're just getting to know each other - you're just learning each other - you both are on best behavior. You don't even know the person after 2 months.

I'd say, IF you need a date NO sooner than 3 to 6 months.

Don't be needy or desperate to push this.