Hi SkinsFan,
How long the wood lasts depends on how high you set the thermostat. The density of the wood, the dryness of the wood. I've had chunks burn completely up on a 5 hour rib smoke, but had some that were only partially charred on a longer jerky or summer sausage smoke.
The key is, and this goes for adding wood to the initial load too, is that it is said meat quits taking on smoke after it hits 140 degrees internal temp.
So, yes, you can add wood as long as your cut of Que hasn't reached 140. Just be careful not to oversmoke when entertaining guests. Not everyone's taste for smoke is the same.
Hope this helped a lil.