In his letter Mr Collins tells them that he can stay for over two weeks "without any inconvenience, as Lady Catherine is far from objecting ..."
Lady C has given him leave to trespass on the hospitality of a cousin whom he has never met before! The nerve!
Mr Bennet comments upon this, saying that he may "prove a valuable acquaintance, especially if Lady C should be so indulgent as to let him come to us again." ;-)