What Larry described is not the way I typically see it. Most of the firms in our area, and the local ERISA attorneys, still charge on an "as needed" basis. That means there is a fee to restate the document, or amend the document, whenever a restatement or amendment is required.
The advantage is that you are only paying for the service when necessary. The disadvantage is that you have a relatively large legal bill every 5 to 6 years.
Larry - under your structure, what if the client leaves you after 5 years of advanced payments? Do they get that back, or is that considered "support"?