Once you have sown your seeds, you want to Farm the tile at least once every real day. You will raise your skill more quickly if you spend Farm more time Farm'ing each farmable tileoften, though about once every 6 hours is about the most often you will be able to.
Your crops should ripen in about three two days but this is not precise. Each tile will grow at a different rate and if you sow 10 seeds at the same time then those ten tiles may ripen individually in 2-4 days. 3 days does seem to be about the average though with the tiles needing to be harvested within about 8 hours of becoming ripe.
Your crops will be ripe when the top of the crop (when groomed) is exactly level with the top of a Small Barrel placed in the tile.
After you harvest you should Sow one seed right back into the tile.
You get very little Farming skill for Harvest'ing and Sow'ing and you will get Farming most skill in the following days when you from Farm the crop and then re-harvest it'ing.
Your best bet is to assume that you will not be making any real crop gain from your Farming until your skill is at 15.