When I first started doing fieldwork, I had to eat lunch. I was ravenous.
Now that my metabolism has slowed down, I either have something small/snacky (a granola bar or handful of nuts) or go without lunch altogether. It depends on whether or not I cobbled together a good breakfast and how busy I am - if I'm running around like a crazy person, I may forget about eating altogether.
If I don't eat lunch, and I'm going, going, going all day, then clearly the entire day is billable to the client. But what if I'm overseeing people who stop and take an actual lunch break?
I generally consider the entire day to be billable. If the field crew is taking lunch, if I'm not eating something (or I spend 2 minutes eating a granola bar), I'm catching up on paperwork, mucking out the trailer, answering phone calls and e-mails I've ignored, etc. It is a slow day indeed if I have nothing to do for a half hour or so.
If the budget is super tight, I may donate that half hour or so. If there's a chance that I'm not going to have at least 8 hours of work that day, I'll probably consider the lunch time to be billable. On a very rare occasion I'll actually Go Out and sit down for lunch with coworkers/contractors, and that time is definitely not billable.
Consultants, what's your lunch/billing policy?