a nice long letter; nice long

Nice long seems to be have acquired a double connotation. The phrase or idiom can be said with a glint of cynicism as in, "Yeah, that'll be there for a nice long time." Or, more literally, in a more congenial sense, "Think I'll take a nice long vacation." However, its use in the Austen quote does seem to hang in ambiguity (perhaps intentionally?).