According to Microsoft, the EOL time for Windows 7 is still to be determined. However, looking at other operating systems, I'd expect Windows 7 PC's to be available for a few more years yet, and since end of support doesn't end for XP until 2014, that's 13 years after it's release. Rumour is Microsoft will continue support for Windows 7 until 2020.
To tell the truth I have to say that I'm not a big fan either, and if I had to run it as it is I probably would not.
But it is faster on my computer and by installing Classic Shell I don't really see a lot of difference between 7 & 8.
I've run it long enough now to feel comfortable with it, and I don't like going back to 7 any more.
When I install the final version the first thing I will do is install Classic Shell and it takes care of the problems.
To answer your question, Windows 9 will probably be out before they really stop support for Windows 7.
I still know a lot of people happy with using Windows XP. The fact that there is no real support or updates doesn't seem to make it unusable.
I believe you can even run MSE in it, and the latest IE isn't that great.
I still use Chrome anyway.
I suppose it will eventually be impossible to run new software in it, but most stuff now will still run on XP.
This is very true, I give some of my customers MSE on their XP's for protection, I'd like them to upgrade to Win 7 or 8 but they feel comfortable on XP. I'd say about 97% of software works on XP because XP was a very popular OS and it dosen't require much to run things. But as for myself I really have moved on from XP, since Win7 and 8 provide a much clearer display with improved clear type and font support. And in general much better driver support, newer desktops and laptops will benefit more from 7 and 8.
There is much choice for people so my advice is to use a Windows you like the most (just not too old or outdated like 98' or ME). And customize around your preference, I personally have taken Windows 8's RP Aero and used it on Windows 7 for a nicer look, and also used a Start button changer so now instead of the default Orb button, I changed it to a Windows flag. It looks very nice, I always make sure to keep up on defraging and all so its always running tip top. But be sure not to install to many updates because some are really not so helpful and may worsen performance, always read the description of what certain updates do. Anyways whatever you choose XP, 7, or 8, I wish you luck.
As far as I understand the furthest support for Windows 7 is for the professional, and it will go to 2020. Considering Microsoft has prolonged the [security] support for XP, I would trust they will extend security support to at least 2020 for 7, probably further. If not otherwise, they will do it due to their Company Contacts.
I find it hard to believe that Microsoft will cut connections at first possible point. Doesn't sound very plausible. Windows 7 will most probably will be supported 'til 2025. Maybe not Home Versions, but who knows?
We need to remember that XP support was extended for some five years, it's still active 'til 2014 concerning security issues. The main issue is probably the one that was with both Windows 98 and XP: corporate customers. When it comes to business, it's about $$, and the amounts are higher / more reliable than those in consumer part. Consumers are actually food, eaten by big-enough companies.
Windows 98 and XP were not extended by popular demand. They were extended by [Big] company demand. Most probably Windows 7 will, likewise, be extended. The simple rule of money: Companies will not pay.