Best rest length for "getting your seat"
There is no one good answer to this question, because there are many personal factors which play in. 

Your weight, your physical fitness, the quality of the saddle, and whether it fits your body well will all be a factor here. 

In general, a short (sub 1 hour) ride every 3 days works well for most people. 

Consider getting rid of your stock seat, and getting something more fitted and more stable for your body to adapt to it. 

I hope that helps.