Most people find that they are reasonably comfortable after a couple of months. It is the readjustment of your brain to living without foreign stimulation that takes a long time. Experts believe that it never completely recovers, because a very few cigarettes years later can result in a full-blown addiction very rapidly.
The nicotine leaves your system within 24 hours, unless you are using replacement therapy (which I highly recommend — the patch in particular, since it does not support the oral habit). There is no point in suffering through nicotine withdrawal during your first few weeks of not smoking. It does not build character; it simply invites relapse.
There are habits that take a long time to get over, as well, such as reaching for a cigarette at certain times: on the phone, after a meal, with a drink, while driving, etc. I was appalled to find myself reaching toward my breast pocket for a smoke during an argument with my wife — more than 10 years after I had completely quit.
Perhaps the most important advice is to keep in mind that EVERY reason you come up with for having a smoke — now, or after you quit — is simply an excuse to feed your addiction. There is no good reason. The weight gain can be dealt with by exercise, and depression and other emotional symptoms, if present, can be supported by Nicotine Anonymous and counseling if needed. (See the link in the sidebar.)