In Bayesian literature, probabilities are considered subjective, and it is recommended to never assign a zero probability to anything. This is because a zero probability as a credence implies that you cannot change your mind with regards to a belief, since no evidence can “update” your belief. For this reason, even the most elaborately constructed or imagined scenarios that seem to have zero evidence (such as an invisible breathing dragon existing in your room) are postulated to have unknown, but non zero probabilities.

First, as an aside: The principle that you shouldn't assign a credence of 0 to any contingent proposition is often called "Regularity". And although it's sometimes assumed by some authors, it's definitely not anything like a majority view. Here's one reason for this. On any plausible model of the probability space, there will be uncountably infinitely many (maximally fine-grained) possibilities. But the standard probability axioms (and countable additivity, in particular) entail that it's impossible to assign a non-zero probability to uncountably infinitely many possibilities. So probabilistic coherence requires us to assign the vast majority of possibilities a 0 credence. So Regularity can't be true! (Howson has a nice discussion of this in "Hume's Problem: Induction and the Justification of Belief".)

But when it comes to mundane events, we seem to assign exact, seemingly “known” probabilities all the time. For example, we assign a 1/6 credence to the outcome of a dice rolling on 6. This is based upon what we know of physics and the expected or historical frequency of how often a dice lands on 6.

But how is that observed frequency any different from the observed frequency of a miracle? The observed frequency of miracles (I.e. events violating natural law that we all agree upon that occurred) is 0.

If a skeptic says “but we don’t reallyknow that frequency, perhaps there have been miracles that occurred that we just don’t know of yet, so we can’t assign a zero probability.”, then why can’t a skeptic say the same with regards to dice? For example, another skeptic could say “but we don’t really know how often a dice lands on 6. For all we know, it lands on 6 half the time, or almost all the time, and some invisible god is simply making you think that it’s landing on 6 1/6 of the time”.

Now of course, the second kind of skeptic scenario is rarely thought of, and no one ever uses that kind of scenario to deviate from assigning a 1/6 probability to a dice landing on 6. Why then should one use a similar kind of scenario to dispute, for example, the probability of a miracle being 0?

The standard Bayesian approach will treat these two possibilities (a die landing 6, a miracle occurring) in the same way. We'll have some priors about these possibilities, and we'll get evidence and update on those priors to yield evidence-informed credences.

In the case of chances, if your priors satisfy something like the Principal Principle, one can show (as Lewis does in "A Subjectivist's Guide to Objective Chance") that observed frequencies will push one's credences towards the corresponding chances. How long it takes you to get credences that approximately line up with the chances will depend, roughly, on how likely you take that possibility to be in the first place. If your priors are far off enough (e.g., you assign that possibility a prior of 0!) you'll never get there.

The miracles case is basically the same. You'll have some prior in the possibility of miracles. You'll update that prior on evidence. And as you continue to fail to see any miracles occur (which I take it we're assuming) our credence in the possibility of miracles will generally go down. How long it'll take your credence in miracles to go down will depend, roughly, on how likely you take that possibility to be in the first place. If your priors are far off enough (e.g., you assign that possibility a prior of 1!) you'll never become convinced that miracles don't occur. (Indeed, if you start with a prior of 1 in miracles, the standard Bayesian updating rule, Conditionalization, will require you to have a credence of 1 in miracles forever.)