Picked up the rules and played a couple of solo games and with an opponent today. I would say the advice that three brigades a side will take about 3 hours (an hour per brigade) is a good guide . The cards and turn sequence keep both sides involved - and you can't guarantee anything :-).
A couple of things seem a little odd from what I've understood so far- but are more observations that could be house ruled than serious concerns. eg Heavy Cavalry intimidation seems to have the same effect on a square as troops in other formations. Also a unit that is disordered and with more disruption than stands that is due to break in the end phase can still launch effective charges eg 6 strong professional cuirassier regiment in column of attack gets 6 die (6 bases +2 shock -1 column of attack, -2 disorder, +1 superior quality) hitting on 3's.
I get the feeling large units tend to do better.
Overall I've enjoyed playing so far, and importantly the non napoleonic opposition enjoyed them too.