Hi wfc,
just asked Martin,for him he started getting dry after 3 weeks, but he did notice that his painkillers made him really dry. Towards the end and even sometimes now on dry days his tongue feels like a dried up piece of moss, I know because he once asked me to touch his tongue.
All I can advise is keep a little bottle of water with you at all times (Martin fits it in my hand bag when we go out), and sip a little but often. Don't drink too much in big gulps, because then as martin learnt you feel to full of water to eat and it sometimes makes you feel nauseus, and the slosh sloshing in the belly is unpleasant.
Anyway, good luck.