The above posts are all good.
Per diem at the airlines is as mentioned above: If your spending the night away from home it's tax free, and if it's just a day trip then it's taxed.
As far as getting furloughed... I guess two years would have been a good ballpark number. I was at ExpressJet (Jetlink) for 16 months and just got furloughed. If I had gotten on 8 months earlier, I wouldn't have been, so your 2 year formula in this situation would have worked.
Also, when doing your budgeting and financial plan for your airline career, let me be the first to say that you will be innacurate with your plan. When I got hired at Express we were hiring 80/mo, people were sitting on reserve for a month, and were being upgraded in 2 years. About two months after I got hired that came to a screeching halt. I didnt break guarentee once my whole time at Express, and I spent the entire 16 months on reserve.
Such is life, and while it's unfortunate for me, my point is that you can't plan very well for what it will be like when your hired there... so have backup plans.
I had also wanted to go to a "good" regional, so I held out for ExpressJet or SKYW (due to base locations). I canceled my Skywest interview and went to Express... now in hind-sight if I had gone to SKYW, I would still be employed and would probably be closing in on an upgrade in the Brazilia..
Also I had a lot of friends who didn't wait for a good regional and they went to Mesa... They're still employed and are also closing in on upgrade.
Making the right decision in this industry is... well... difficult. Do your reasearch, like you are, and make the best decision for you at the time.
Lastly, being gone for days at a time does wear on you. I imagine at a major it could be fun going to neat international locations, but they're is nothing glorious about overnighting in Beamont, Brownsville, Midland, etc etc. =) Ya make the most of it while your there, and have a few beers if time permits. Just make sure your ready for that... a lot can happen in 4 days to friends and family, especially if you have little ones running around.
And very briefly to your origional regional vs major thread... as mentioned people stay at the regional because they've become comfortable with their salary and quality of life. holding 16 days off a month and making 75k is good for some people. Other people want the "stability" of a major, or to fly bigger aircraft, or to go international. Other people are happy flying a king air to the same 5 airports their whole career. I know I'm giving the extremes here, but I'm just trying to widen your view of aviation.
As everyone says.. If I could get paid 100k / yr to fly a 172 and have job security, I probably would. =)
Enjoy the rollercoaster.
-Travis