This paper provides research on exactly what you're asking about (note: link opens a PDF). I have attached part of their conclusion below:
First, while we ﬁnd
that the credit rating agencies often disagree, it is usually conﬁned
to one or two notches on the ﬁner scale. Second, rating transition
probabilities tend to increase as the rating level decreases across
all agencies, but rating stability at lower rating levels is less for
S&P than for Fitch or Moody’s. Third, we document that six variables are common determinants of all three agencies assessments
of credit quality. However, the fact that a further four variables
have varying importance across agencies leads us to conclude that
material heterogeneity exists between the agencies. Fourth, our
hazard and ordered probit models both suggest that watch and
outlook procedures are generally strong predictors of rating
changes relative to other public data. S&P outlook data provide
the strongest in-sample prediction performance of any agency-
based rating forecast, but Moody’s and Fitch watch data outper-
form the prediction performance of S&P watch data.
In short, rating agencies do disagree at times, but they are generally very close to each other. Ratings tend to change more as the rating level falls across agencies. Ratings outlooks are a good predictor of future ratings changes.
Denmark, Finland, Canada, Australia, and Sweden have all been downgraded from Triple-A before but have recovered.
Canada waited almost a decade to regain its triple-A status after being downgraded, while it took Australia 17 years and six consecutive surplus budgets.
Source: Credit Ratings Comeback Kids
Agencies regularly evaluate their ratings, but when it comes to sovereign ratings, history shows that it takes a long time for countries to regain triple-A. Estimates suggest that it'll take the US at least 10 years, but it might be faster, or slower. We don't know - it depends on how the recovery goes and how America manages it's debt.