Learning Finnish like any language is definitely possible. There is a sort of maths to the whole thing.
It's more-or-less: Time + Effort - Memory retention = Speed
For example:
If you come here and jump into a course you are spending a lot of time learning, a lot of effort then minus your memory retention. Going to school constantly for 2 years one could definitely learn enough Finnish to be able to survive on one's own here. Hell you could probably make do with just a year. Some of course make do without knowing ANY Finnish. Though it would certainly, without a single doubt make their lives quite a bit harder when it comes to having to actually interact with Finns. There is no real rule of thumb for private vs. public courses. Normally you have to do public ones unless you can get Työkkäri to accept your private course. Though good luck doing that... One can always do both though. Public courses are in pretty bad condition but the teaching should be pretty good just as long as the teacher doesn't take any crap from people like the idiot above me.
In my opinion it's bad to force a Finn to speak your language regardless of him/her knowing it or not. As it has been said a million times. Finns may know how to speak English but whether they will want to is another problem entirely.
If you were to not take courses here, public or private. Then your learning time would be a lot lower, your effort would surely be a lot less because you would be working or something else. Then minus your memory retention which is less when you are not being repetitious. This means that it will probably take literally DOUBLE the time. I have seen some people here who have spent only 2 years, who can more or less speak Finnish well enough to get their point across, understand most of everything and definitely get through day-to-day life. Things like going to the doctor are still a bit of a task for them but if one kindly asks the doctor to speak in a bit more 'foreign friendly' terms one shouldn't have any problems.
Learning Finnish is possible. There are thousands of people who attribute proof to that. The poster above me is plain and simple just a troll with a vendetta towards everyone in this country. In retrospect he should piss off back to the sandy cave he was undoubtedly born in. I for one, if I was him would thank my lucky Allahs for the generosity that Finland let him in, in the first place. Let alone the mods who let him post his flaming garbage in these forums. Then again, you can lead an 'aasi' to water, but you can't make it drink.

The rate at which you learn Finnish will ultimately depend entirely on your determination and seriousness.
Some learn ridiculously quickly. Others don't know a thing after 10 years. Some unfortunately are born speaking another language that is so different from Finnish (Chinese, Zulu, Vietnamese etc.) that they will probably NEVER be able to speak proper Finnish. Or at least in remotely the kind of a way a Finn could simply because their pronunciation is different and they hear sounds differently.
Your quest of learning Finnish is up to you. Don't let anyone tell you otherwise, especially this idiot above me who is crying out for attention so badly.
Learning Finnish isn't impossible but it's darn hard, and that just makes it all the more rewarding in the end. Like all things there is a learning curve.