"Hire slow and fire fast". I've never seen a company that had not made several bad hires. The difference was the speed at which they were willing and able to identify them and remove them. You shouldn't trust someone the moment you bring them on, but I do believe that should be able to imagine yourself trusting them.

A general rule for me is that you should know within the first 2-4 weeks whether you've made a bad hire. This is a bold statement, but my experience is that this time frame will give you enough time to get a gut check (super vague, but there are too many different patterns for me to articulate here). If your gut says no, that will stick with you, and your gut will likely become a self-fulfilling prophecy anyways.