Days of supply:
SPR: ~65 (add 60 to all below)
With domestic production that extends this supply out another 2 months. However, minimum operating levels exist with stocks of oil products, meaning fall below MOL and oil product deliveries become highly erratic.
When you include Mexican and Canadian Oil production, and also Saudi and Iraqi Oil production that is imported, the U.S is more than well supplied. Those four exporters will always form the basis of U.S imported oil. If those 4 and what we have now in the SPR and other stores was all we had, we could go 1.5-2.5 years (depending on demand, and production) before major shortages became problematic for agriculture and civil order. Rationing could extend those few years out another year or so before that point was reached.
In terms of MOL, we stand only 20 or so days away from major shortages, and by the time most tanks ran out of gas (gasoline demand is down because people are running their tanks dry) a day or so later, and food pantries had been raided, a month after MOL was breached people would start running out of food in most major urban areas. This is why its so important to have a nationwide plan if something like Katrina happens again, because something small could effect the entire country.
That said, with the level of supply we have (in terms of days of supply, not physical quantity, because demand is higher than it was 15 years ago) is quite great relatively to the last year, when shortages looked very probably and which we avoided by the skin of our teeth, literally. If we make it to 2010 without major shortages of any sort, new oil projects, ethanol production, natural gas powered vehicles and economic decay will keep oil demand within supply boundaries, meaning society won't implode, and society can cope more easily with more expensive oil.
If we see shortages now, before Wind Power generation has reached critical mass, before Coal to liquid projects have been started, before Ethanol production doubles again, etc, then all of the industry that it will take to make those things will be ravaged and it will make the next decade much harder. Remember, industrial capacity must be able to function for us to change off of Oil and to more reasonable sources, like renewables, NG, solar, wind, ethanol, coal, and nuclear. The current financial crisis is not helping, nor is the grandstanding by Mr. Iran and Mr. Chavez. They literally can screw everything up and put is in a depression. Thats why war is sometimes the answer, and if you don't fight it, well, you end up in a depression begging for handouts from Madame President.