Drawar

I always thought it depended on the printer and the surface you are printing to, but seeing as it's a question, I'd say 200dpi. If that's not enough then go for 254dpi, if that still will not suffice then go 302dpi.

72 isn't a standard dpi it's a standard ppi. Dots do not equal pixels -- one is a measure of printed dot density and one is a measure of screen density.

To be safe I'd shoot for 600 dpi, as you'll probably end up using a laser printer. Maskau's right about the number 200, but use that for ppi not dpi.

So 12" x 24" @ 200 ppi = 2400 x 4800 pixels. Make sure your image is at least that many pixels big, bottom line. To be extra sure shoot for 300 ppi (3600 x 7200 pixels).

More info here.

I would suggest 300 ppi, as a basic beginning. If you don't like the way it looks, then go higher.

I'd check with your print shop.
Often times large format printers require different setup than smaller sizes.
Your print shop can tell you how to best setup a file for their equipment and they will know how to get the best output from it.

Printers are finicky beasts and sometimes have their own personalities. I speak from experience running a Fiery front end that had a tendency to make everything blue come out purple unless tweaked.

72 is good for display on your monitor. In print it's a very bad idea.

I usually assume 300 ppi for print, but when I got my business cards printed, the printer mentioned that they could take advantage of 400 ppi so I went for it.