Trust is about vulnerability. Once we 'give' trust to someone, we are tied to that entity (trust) being self sufficient. There's little space for 'grey'... we do or we do not trust...we can't 'trust a little'.
But a relationship lives in that vulnerable space. We are tied to a person we trust (especially in a romantic, caring, monagamous [sp?] relationship) by a bond that is supposed to be special and unbreakable. But to be honest, the only bond I know of in my near forty years on this earth that is unbreakable is the love of a parent for a child. No matter what a child may do in their life, no matter the troubles, missed lessons, their inability to see life in a different way.... parents will always love their children (yes there's likely exceptions to this as well).
We are assuming a lot by asking the question on trust. We assume we, as adults, understand what trust means, how you attain it, how you maintain it. What we don't know about another adult is the way they learned what trust means, feels like, how to give it, how to maintain it. Therein lies the crux of the answer. For if we could see how trust was maintained - for people growing up, it may be "I trust that guy with my life because....." and it could be one instance in their life where one had protected, saved, looked out for etc the other. And that could be enough. Maybe trust is about an infinite time span.
For people in a relationship with a partner, one who questions "I trusted you and now you've done this...." Maybe trust is conditional 'if you do this...., then... I pull back my trust'.... So the whole idea of fully trusting someone is confusing in definition as it is in a person's ability to define it and maintain it with others.
If I say I trust someone, I need to know they are aware of what that means for me. Expectations of others abilities to surmise my definitions, exemptions, exceptions... is a little much.
And, it again goes back to vulnerability. Am I so not ok with what they have done that I will agree to let go of all we have built so that I can prove the point "It is not ok"? Or is the vulnerability of continuing a relationship on the premise we as a couple figure out how to get back what may have lost. If the belief is that all is lost on the action of one individual... then... there may not be any chance to get anything back. If the belief is trust has an infinite life span, then, regaining and maintaining it may not feel so daunting.....