House thermostat is set to 68-70 during the "winter" and 78-80 during the other 11 months of the year (I live in Phoenix).
Most of my wine $$$ goes towards inexpensive ($8-$15) daily stuff, with the occasional bottle in the $20-$30 range that according to tasting reports done by WS and the like might be best to hold for 3-5 years.
I do have a small (30 bottle) wine unit that I keep my "better" bottles in, but am interested in buying more daily stuff.
At the temps noted above how long can I expect stuff to last without going bad? Storage area is dark, and vibration free.
Thanks for the help.

Temperature fluctuation is a problem. At those temps, particularly the summer heat, I would say no more than a year or so.

I'd buy a bigger cooling unit even if you have inexpensive stuff if you live in arizona, temperature fluctuation and humidity are bad for any wine.