A quick glance at RPGamer's (non-spoiler) review suggests 15-20 hours and that it feels more like an actual stand-alone adventure than an add-on to DA (which fits with prior BioWare efforts a la NWN.) They rate it 3.5/5, and I typically find that I would score a game a bit higher than RPGamer does.

One caveat: someone I know who played DA:A says that the character that carries over from it to DA2 has completely changed personalities in the meantime.