In some web sites I see listed various "default rates" for different types of bonds, AAA, AA+, etc.
Are those default rates per year, or over the life of the bond?
I am not entirely sure if you are asking about
I will just try to elaborate on above:
Ratings are tied to the likelihood of the company going default, or the Probability of default (PD). PD is exponential with AAA having a PD in the ballpark of 0.01% (or similar) and C being maybe 80%. PD is usually expressed as the probability of default within 12 months. So a PD of 1% would mean that there is a 1% chance that the company will default within 1 year.
Default ratios will relate to how many companies in each rating class that will actually default. This will be in percentage, not related to periods.