Do not cover the seeds; they need light to germinate. They should sprout within 10 to 15 days at room temperature or slightly warmer (68 to 75?F). Transplant into the garden or container when they have at least 2 sets of true leaves. From seeding to maturity, mint takes about 90 days. This means that it will achieve its full height, generally of 1 to 2 feet, and it will be ready to harvest. At this point you can cut mint down to 1 inch above the soil, and it will regrow to harvest height again in a month and a half or so.