The answer varies for everyone because eyebrow hairs are different. Trimming a brow hair too short can mean the hair will not lie flat. Brow hairs that are fine can be trimmed shorter than coarse thick brow hairs. The heaviness of a longer hair will help the hair lie flat.

For coarse hairs. Best to pick a single coarse eyebrow hair and use it as a test. Trim it down a bit at a time. Note how long it is and yet still lies flat. Look closely at the length of this hair and keep all your brows this length.

For fine (soft) eyebrow hairs. Brush the hair up from the bottom. Trim the length of the hair such that it is at least 3 - 5 millimeters above the top of the brow line.

No matter which option you pick - do one hair at a time. Go slowly. Trim a tiny bit and reassess. Plan on taking 5 minutes to do a good trimming job.