buy+long+years+of+repentance

In this context, buying long years of repentance most likely means giving into sexual desire knowing that deep regret will follow. However, Mary Montegu was born at the end of the protestant reformation. She could be referring to the corrupt practices of the old Catholic church. Before the protestant reformation, one could be absolved from sin if he payed the Church enough (prices depended on the sin). So Montegu's refusal to buy repentance could also be in reference to this practice as well.