Your level is based on the number of points your virtual stable earns during the current month.
The levels are:
NEWBIE: up to 200 points
AMATEUR: 201–450 points
SPECIALIST: 451–750 points
PROFESSIONAL: 751–1,200 points
EXPERT: 1,201–3,000 points
MASTER: 3,001–15,000 points
LEGEND: more than 15,000 points
Your points reset to zero at the start of every month.