If I start doing Vipassana now, how long will it take me to achieve Nirvana? Call me an over-enthusiast but presently, the highest priority in my life is to achieve the object of Nirvana or moksha. By performing Vipassana, I want to achieve Nirvana and in this lifetime only. I'm prepared to do anything for it, even leave my current profession and staying as a monk if required.

Trouble is, I just don't understand how exactly to go about meditation. I've read a few suttas but I just don't understand what to do while doing Vipassana meditation. In a 10-day Vipassana course I'd undergone, I'd done Anapana (breathfulness meditation) by keeping my mind on the entry and exit of breath at the nostrils. I've done this for several hours and I still do it sometimes.

I also do mindfulness meditation sittings (by observing my thoughts impartially) for at least an hour in one sitting daily.

Now, all these practices are great and take me into a calm and tranquil state of mind. But I don't seem to be getting anywhere towards my ultimate goal of Nirvana. Is there any way to know how long is it going to take me with this speed, or how do I increase the speed? Most importantly, Is joining the sangha and practicing full-time monkhood the only way to achieve Nirvana?
There is a lovely zen saying, a dialogue between new student and master:

"Master, if I put in great effort every day to attain awakening, how long will it take?"

The master replies, "20 years."

"What if I work really really hard?"

The master replies, "40 years."

Wanting enlightenment is paradoxical, because enlightenment is the absence of defilement, not the presence of need.

It would be of great benefit to study the six perfections.  You may find this reference useful: http://www.rigpawiki.org/index.php?title=Six_paramitas

Often people assume enlightenment to be the result of something. If you are happy following causes and their results, then practice the six perfections as much as possible.  You must ultimately pair them with wisdom which transcends conceptuality in order to arrive into a state which is beyond.

It will greatly depend on your attitude and your motivation.  Thus, I encourage you to study the Buddha's words and keep a superior motivation.

No one can guarantee for you how long it will take, and when you "get there" so-to-speak, there will not be any conventional sense of time to speak of, so instead of asking "how long will it take" we should all ask "what can we do to accelerate the process?"

A skillful question is one which illuminates the how.

If you come with great faith in the teachings and practice with a single-pointed mind, freedom shall dawn.  Until the gravity of our imperiled situation in samsara is seen for what it is, benefit yourself and others with study and reflection.

One aspect of enlightenment is freedom from doubt.  Clear away all your doubts with the stainfree mirror of the teachings.  The Buddha proclaimed that "in the future" [in this era] to come face-to-face with the teachings is the same as coming face-to-face with the Buddha.

At any rate, even if someone told you that it will take 3 long aeons to come to awakening practicing diligently every day all day, what is that in comparison to an unending cycle of dying and being born?  What is 3 long aeons in comparison to the nonceasing spin of the wheel of life?