I would like to know when is the best time to begin severance discussions? I am a senior level executive who has been offered a very nice position. While I do not see myself leaving this firm before retirement- the possibility is always there.

The best time to bring up severance is before accepting an offer. The secret is to depersonalize the issue. Don't make it about your not doing well with them.

Tell your prospective boss that if you were certain he would always be there and always be your boss you wouldn't need to bring this up. But things do happen and you want to make sure that you are trying to plan for the long term.