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 Farm more often, though about although once every 6 hours is about the most often you will be able to.
Your crops should begin to ripen in about two days later at the same time as you planted them 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.
Your crops need to be harvested within about 8 hours of becoming ripe.
If you Harvest early you will get reduced yield and if you Harvest late you will get reduced yield. If you are only getting one potato, or whatever, anyway then you will get nothing.
After you harvest you should Sow one seed right back into the tile.
You get very little Farming almost no skill for Harvest'ing and Sow'ing and . You get most skill from Farm'ing and maybe a little less than that for Harvest'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.