The closest to a standard I've been able to find comes from looking at front-of-lens filters for solar photography - that is, filters for use with telescopes (or telephoto lenses) specifically for taking white light pictures of the sun.

For visual solar observation, the standard is ND5.0. Baader Planetarium also do an ND3.8 version for high magnification imaging use (NOT visual), and the Wikipedia entry for Neutral density filters has the following note:

Note: ND 3.8 is the correct value for solar CCD exposure without risk of electronic damage.

But doesn't have a citation to explain where the number comes from.

On the other hand, there is a clear conflict between this and everyday experience with non-DSLR digital cameras. It's not unusual to have the sun in shot - either intentionally, or while framing - and yet we don't get burnt out tracks across the sensor from doing this.

Part of the explanation may be due to consumer cameras having built in IR blocking filters for colour balance (Many astronomical CCDs don't). Another part may be that many point and shoot cameras use relatively small lenses - so they don't capture anywhere near as much heat and light as a telescope, which usually have objective lenses or mirrors of several inches in diameter.

Also, with conventional photography, at least for handheld shots, you're usually looking at durations of a few seconds at a time, often moving as you're framing the shot - so there's not normally much time for the heat to build up. If you contrast that with solar imaging, where you're normally tracking the sun for several minutes (or hours, sometimes), with a much larger objective lens / mirror (and so captures more light/heat), you can see why a filter becomes more important.

With a total solar eclipse coming up soon in the USA, it's probably worth adding a safety warning about filters for visual use. You only have one set of eyes so don't take chances - only use filters specifically designed for solar use. Don't try to improvise from exposed film, CDs or whatever. Visual performance is not a safe guide for performance at invisible IR / UV wavelengths. Proper solar filters are relatively inexpensive - a couple of dollars for something handheld that you can look through with the naked eye, or 20 - 30 dollars for a sheet of solar filter film, or you can get premade filters (The visual grade ND5.0 ones are fine to photograph through, too).

and as the wikipedia ND filter entry also notes:

Note: ND 5.0 is the minimum for direct eye solar observation without damage of retina. A further check must be performed for the particular filter used, checking on the spectrogram that also UV and IR are mitigated with the same value.

Purpose built solar filters are safe. Others are unknown, and not worth risking your eyesight over.