The outcome is to chat with your farmer to find out which sprayings are toxic to bees and which are not (all sprays have to have this info by law). We then get a call from the farmer to let us know when he will be spraying and we lock up the bees the night before (and hopefully your farmer will be able to spray early in the morning). We then let them out when the spray has dried – stand back when you take the blocks off as they can be very, very stroppy!
I would try to cover off as wide an area as is sensible, but the closest few fields would be a must. I guess if your hive borders another farm, it would be worth talking to the other farmer as well.
Having said that, another view given to me (by a bee inspector) is that if the farmer sprays early enough in the day (especially if early on in the year) the bees probably won’t be flying much anyway, so you could be a bit more relaxed about it. I haven’t been brave enough to follow this route though.