I have lived in Texas and Louisiana for over 60 years where freezing temperatures are relatively rare but I have seen temps as low as 14 degrees and that was during a week where the high daytime temperatures never got above freezing. Also I am a Chemical Engineer which is mainly heat transfer and material handling.
Commercial covers are typically effective for temperatures down to 25 degrees F with above freezing temps during the day. For 23 degs and below additional measures are necessary. What I do is use some cheap Styrofoam coolers and place the bottom of the cooler over the water bib and commercial cover and then brace against the wall as tight as possible with something. This more than doubles the insulating value of just the commercial cover with the additional benefit of trapping a large bubble of air between the cooler bottom and the commercial cover and now you have two air chambers to add to the insulation value. Air if contained and still is the best insulator of all - much better than the Styrofoam alone. Only a vacuum is better.
Now the guy who put a small incandescent bulb under the cover should be good for just about anything. Most of use can't do that.
As far as how long the insulation can keep pipes and bibs from freezing, that depends on many factors, primarily
- the quality of the foam - the lower density of the foam the better as long as you have the structural strength to hold its shape
- the quality of the seal to the wall - any air leakage bypasses the foam insulation and that is bad
- how much heat is leaking through the wall - most homes in the south are not insulated as well houses in the cooler areas so we get a small but appreciable amount of heat through the outside walls which helps.
The advantages I see in covering a commercial cover with the bottom of foam cooler is that you get more thickness of foam over all and you have two trapped air pockets and with the cooler you cover a much larger area of the outside wall and therefore get more heat from the house.
However except for the heat you get through the wall of the house everything is passive insulation which while it can effectively slow down heat loss it cannot prevent it so if it stays cold enough, long enough your faucets will eventually freeze. Where I live that limit is at least as low as 20 degrees over night with the double covered faucets.
After that you have only three options
- add some heat inside the covers like a low watt incandescent bulb
- cut off and drain the bib
- use the stand off type bib where the water is well inside the wall
Options 2 and 3 require a lot of work ahead of time but if you are caught with an unexpected freeze you best bets are insulating covers and double covers.
One last options people used to do is drip the faucet so it does not freeze. There are number of problems with that
- if everyone does that the area water pressure may be so low that for all effective purposes you loose your water supply
- if you don't drip enough, the drips will freeze and then faucet, especially a problem if more people start dripping during the night and the pressure falls.
- even if the dripping does not freeze, you may find a small mountain of ice under the faucet in the morning and in one case I personally saw the ice actually grew up to the faucet and restricted the flow enough to cause the drip to stop and freeze the faucet.
Dripping is better than nothing but is probably to be used only as last resort.
If you don't have a commercial faucet cover or a foam cooler, use lots rags/cloth and tape and then put a plastic bag/box over it and tape it tight against the wall - trapping air and isolating precipitation from the rags.
One last note about freeze damage of pipes and faucets. Water freezing is not the problem. The problem is unlike most other solids which shrink the colder they get, water shrinks from about 32 deg F and then around 28 degrees starts to expand. The expansion is what causes the damage. In most cases even if the temperature gets to 28 degrees and the pipes freeze it almost always does no damage. The damage is typically done when the pipes get below 28 degrees. So freezing temperatures is something not to panic about, but the hard freezes below 28 degrees are.