As a host for 3 years we always make sure that we tell our wwoofers what is expected during their stay so that there are no surprises. Wwoofers should always ask, if not told, what is expected. 6 hours a day seems to be usual. We ask our wwoofers to work 6 hours for 5 days out of 7 for full board and the accommodation we offer is in a guest house. Some hosts only provide one meal a day, others have tent accommodation off grid. Each is different and there are no limits, which is why its important to understand expectations beforehand. Its an agreement between host and volunteer. If the wwoofer feels they are being exploited then first they should talk to the host and if this fails to reach an agreement then the wwoofer can always leave. The wwoofer can also report the situation to the host country's organisation.
With regards to the type of work - we have also asked wwoofers to do non-farm work on occasions and so far all have been happy to do this, I guess it depends how its asked.
On the other side of the coin we have had wwoofers who's work rate is very slow, e.g. a weeding rate of about 1m2 per hour. We find that work rate goes up considerably when we are working with the wwoofers rather than point them to the task and leave them to it. We have also had a wwoofer who could not get up before 10.00 and was on drugs - we asked him to leave!