It takes a minimum of 6 weeks on an average to achieve a noticeable change in cardiovascular fitness. Less fit individuals see progress even more rapidly and for more active individuals more time is needed. An average change in cardiovascular fitness for adults is 12-15% in 10-12 weeks if moderate intensity exercise is performed 3-4 times a week for at least 30-40 min each time.
The purpose of Polar Fitness Test is the same as that of all fitness tests; to monitor this progress. It is not so much the exact OwnIndex values, but rather the trend, that is the most important thing to follow.