I haven’t used Rosetta Stone, but French is basically the easiest language to learn (I did French in HS and Japanese in college). Having said that the general rule is to carry on simple conversations you need to put in roughly 500 hours. So if you did 3 hours a night in French you would be ready in roughly 6 months.
I have taken French, Spanish, and Russian, all in college and I found French the hardest. It will be especially hard if you do not have any experience studying one of the romance-languages. DO NOT underestimate how hard it is to
really learn ANY language. Especially as an adult. You have to really commit and totally dive in if you want to be able to say anything beyond “My name is >> and I am from America.”
I have had many personal classes and online classes, but not Rosetta stone. I spent years studying Russian in college and it was still difficult to speak. One thing you'll learn is that comprehension will come first. After lots of studying, you'll be vaguely able to understand a few words someone is speaking to you, and remember that natives speak
FAST, not slow like they do on most apps and classes.
I recommend whatever system, app, etc you go with, you use a combination of things to REALLY learn. Watch and repeatedly listen to any French tv shows, radio programs, or newscasts, so you will hear real speakers, speaking actual-speed French. Doesn’t matter if you don’t understand what they are saying, your brain is soaking it in to some degree. At the same time, start drilling vocabulary such as nouns, verbs, adjectives, etc. Rinse and repeat. It is quite the commitment and most people get discouraged after just a few hours of studying. Believe me. Really speaking a language takes years of practice, but is well-worth it IMO and a very rewarding experience. I always laugh when people say they speak 4 languages, but only speak about 10 words of each. Just practice, practice, practice, then practice some more. Good luck!!!