That is not currently an option. You get from 30 seconds to 5 minutes, and never is a selection. However, I could not believe someone would want never, considering the security implications. If you do not see Never as an option on your device, do you have a work email on your device, or is it managed by your employer? That could be a reason you do not have Never as an option.
If you do see the other settings I described above and wish to see a change, see this. If you wish to provide feedback to Apple regarding a change you would like to see, a change you didn’t like, or report a bug, then use the feedback link here, Feedback - iPhone - Apple