Heat Generation by Given Size of Parabolic Dish

Look up insolation (at your latitude) it should give you a figure for watts per square meter.
If you assume your parabolic reflector collects all that falls on it (∏r² for area), factor in an efficiency, say 80% at a blind guessif you like.
Then you just need the specific heat of water, someting like 1 watt raises 1cc by 1 degree C or some such (I can't remember an I'm sure as heck not googling that for you!).
Google to get the real figures and do the arithmetic.
Del
(Then let us know the answer)

If you just want to boil a few liters of water, a fresnel lens would be a less expensive and easier method of doing that. You can google 'fresnel lens' and find a number of sources to buy large ones for doing that and pages that talk about solar concentrators. You can go to Wikipedia and search on 'fresnel solar concentrator' and find a page that does the math.

And there is also this cool video:

http://www.youtube.com/watch?v=njs2jecuuRU