Full Idea
A mental state cannot count as a belief unless it has a reasonable life expectancy. It must be the sort of thing that one is disposed to assert not only now but in the future too.
Gist of Idea
It is necessary for a belief that it be held for a length of time
Source
Miranda Fricker (Epistemic Injustice [2007], 2.3)
Book Reference
Fricker,Miranda: 'Epistemic Injustice' [OUP 2007], p.52
A Reaction
There are obvious counterexamples, where a firm belief is strongly formed, only to be dashed by a counterexample (such as a new witness in court) soon afterwards. That said, this idea is obviously correct.