IMO, misspoke could be used when someone says something that at the time seemed true to them, but after further thought (or argument) it becomes obvious, it wasn't true.

'misspoke' can mean something said but not in the way it was meant, as well...but I consider that usage more of an excuse and the 'mispeaker' just wants to be let off the hook.

How long before you trust? Depends. If it was a true 'mispeak', perhaps when anything else is said on a topic you're conserned about, I would ask for clarification i.e., repeating the statement, if for no other reason, than to make it unavoidable for the one 'speaking' to say anything other than what they meant.

If it was the other...just an excuse because of being caught in a lie....the answer would be never.