I arrived an hour before the box office opened, and was among the first 5 people on line....no problem at all getting rush tickets (which were excellent).
This was during previews, and I don't know if the situation has changed since then, but an hour prior was more than enough time.
THE INHERITANCE is well worth seeing. The word/hype from London was SO extremely positive that I feared I would be disappointed. But I loved it....and have found myself thinking about it often over the last weeks. One of those plays where I like to read the script and really delve into it.