Most D1 coaches have walk on stds. Typically 4:20 / 9:20. Maybe faster or slower depending on how competitive the school is. Additionally, some schools have time trials, but they are not as common and the standards are all over the place.
You might encounter more resistance than Freshman simply because the coach ultimately wants athletes that can score at conf, regionals, ncaas. So, they're typically more willing to give Freshman more leeway because they have more time to develop (and they're evaluating them based on high school times, so there is an assumption that, with high volume and intensity training, that there is significant room for improvement).
Look at it this way--what if you wanted to walk on as a Senior? You better be running times that would score points immediately or the Coach shouldn't give you a spot.
So, my advice would be to talk to the coach, know the standard and beat the standard by some significant margin. Better advice would be to know what it takes to score points at conf, regionals, ncaas. Work toward that as a goal. If you don't think you can reach it by Junior year, run club.