Just like 737Pylit said, I would seriously consider whether or not you really want to pursue this as a career. Flying is fun, and it'd be great to be able to do it as a career, but you can always do it on your own. There are certainly a lot of things out there that are arguably more important than flying (family, friends, financial security) and being an airline pilot isn't very friendly toward those things.
The airline pilot career is not what it used to be. I don't know that we will ever again see airline pilots making $300,000/yr. You will be lucky to max out at $150,000 after 10-15 yrs with an airline. I would not be surprised if those numbers decrease even further as airline pilots become more and more of a simple commodity. I know plenty of people in my neighborhood who make significantly more than that doing lots of other things. One guy works from home as a mortgage broker, another guy works independently in pharmaceutical sales, another guy is an exec for the software arm of a big company. They don't have to sit reserve and they are home pretty much every night and I also see them home frequently during the middle of the day. Things like that may not seem like a big deal to a young guy, but believe me, they become progressively more important the older you get.
A friend of mine during college wanted to become a dentist. I remember at the time thinking to myself rather pompously, "How boring! I'm gonna be a pilot and it'll be way better than being a dentist." Tell you what, that guy did his 3 or 4 yrs of dental school after college, worked with another dentist for a while after that, and has now had his own practice for several yrs. Today, he has a REALLY nice house in the nicest area, goes on vacation (Costa Rica was the last place) for a week or two every couple of months, and is essentially stress-free. Me? I'm stressing. Oh, and BTW, according to him, being a dentist isn't boring.
The airline business has been historically unstable. One airline exec was quoted as saying, "If God had intended airlines to succeed, he would have made them profitable." What that means to you is that you will never feel secure in your job and you very well will endure at least one furlough and/or liquidation. I read somewhere once that, on average, airline pilots are furloughed 2.5 times in their career. Think TWA, Pan Am, United, ATA, Eastern, Braniff, USAir, American among many, many others.
The cost (money, time, energy) to become an airline pilot is relatively high. This applies most directly to the question you asked. I'd say it compares with what a doctor goes through to become able to practice. It's a huge apprenticeship program. If you go military, that means 4 yrs of college, followed by about 1.5 yrs committed to pilot training, followed by a 10 yr active duty committment. You might get hired at that point. Or, if the industry is in shambles when it's your time to get out, you might have to sign for more committment or get out and become a Reserve/Guard "bum". The civilian route also seems difficult. I'm not as familiar with it, but it sounds like it is very expensive and requires you to scrape by on near-poverty level wages for at least several years. It might work out to be quicker than the military route by a couple of yrs. Whichever route you go, you will likely have to devote a large amount of money to the major airline job search (airline tickets, memberships, hotels, cars, interview suit, type rating(s), interview prep, convention tix, books, etc).
As an airline pilot, you are also in jeopardy of losing your medical. If a real estate agent, software developer, or banker develop a heart condition, they don't lose their job. You very well could.
Also, as an airline pilot, if you switch companies (whether by choice or otherwise), you start over from the bottom unless you're lucky enough to get a blended seniority list during a merger or buyout. I can't think of any other industry that requires that.
There are advantages to being an airline pilot. There's the view from 350, the absence of office politics, and doing something you may love or like. Do those pros outweigh the cons? I'd say it's something to think about very, very seriously before you embark on the journey.
As far as how many hrs are required, it all depends on the timing of the industry and the particular airline. The hrs that would have gotten somebody hired five yrs ago won't even get someone close to an interview today. If you're interested in particulars, go to the airline websites. Look for the "Careers" or "Work for Us" link and see what the airline requires. Typical mins are 2500 total time and 1000 PIC. Often, a significant portion of those times must be turbine time. Keep in mind that those are official mins. Competitive mins could be considerably higher.