I've heard of many people successfuly getting one of their revolving credit cards after 6 months of a charge card.
What is your target card? For higher end cards like Blue & Clear I would wait another 6 months.
mjbfan, did you get your AmEx this year? (in 2010)
If so, wait until January 2011 or later.
The subsequent card will have the month that you open it + the year in which you opened your first AmEx.
If you get a second card this year, it will show up as a new card. For instance, if you get one next week, it will show September 2010 for the opening date.
But if you wait until January, the new card will show an opening date of January 2010, so you'll have instant age, and you won't get a new card ding.
If you got your current card in December, say, of last year, then getting a second card next week will have an opening date of September 2009.
Might as well make the AmEx backdating work for you!
For anyone else reading this who doesn't have an AmEx yet, think about getting one in October-November-December, so that you can work the backdating to your advantage with subsequent cards. If you get your first card in January or February, you'll have to wait a year to get that instant age working for you.