As Bert Preast indicated, you can increase the skill three times a day (with the "day" starting and ending in an otherwise unmarked time during what's described as "morning". If you know you had a skill increase the previous day, you can check for stars to determined whether the day has ticked over yet, otherwise wait for "late morning"), but the star indication appears on the first increase, so you'd have to keep track of what the skill was before the increase to know how much it has increased (and thus whether any additional training has any chance to actually produce any results).