I've been on quite a few SSRIs (as you can see in my signature) and you couldn't pay me to take another. I'm still supposed to be on 40 or 50mg of Fluoxetine (trade name Prozac), but I took myself off of it without telling my pdoc.
You start feeling the effects of an SSRI about 4-6 weeks after you start taking it, unless your brain has been exposed to an SSRI in the past, in which it will only take about a week for a new one to kick in.
Not sure what you mean by "how long" you should take it, though. Elaborate a bit?
Hope I can help