Don't look at it as you're giving HIM an ultimatum. As I said, I would not even convey to him what your timetable is necessarily (only that your "patience isn't unlimited"), so how can that be an ultimatum?
No, it's setting a timetable FOR YOURSELF. YOU need to ask yourself how long you're willing to commit to this, and you need to convey to him that you're no longer willing for him to cake-eat. This will have the simultaneous effect of making you feel better about YOURSELF, and also make you more attractive to him, for standing up for yourself.
You have answered yourself in your first paragraph, Louise.
Choc.
P.S. I'm with Oden on the prayer issue. Ask God to remove your husband's pride, and his sense of entitlement, and to open his eyes to the pain he is causing his family. Ask God to send people across his path each day that will cause him to have second thoughts. I prayed this, DAILY, and my wife told me the night she came to me asking to reconcile that a stranger to her that day had told her that she was married "22 years" (the same # as us), and that "he's my home," and something in my wife just melted. She said there'd been "a little voice" in her head all along, telling her that she she was doing was wrong, but that she'd always been able to blot it out until that final week, when it had gotten louder, and louder, and LOUDER still.