A Unit ordered to guard a region will do this starting the following week for an unlimited period of time. It is much simpler this way, because in most cases you will want to keep your regions guarded all the time and you don't have to give the command every week. The guarding status is lost only when you explicitely order the unit to stop ("GUARD NOT") or move the Unit to another region (and probably also when you disarm it).

The manual states (relevant passage emphasized): "Units can guard regions if they are armed and have the appropriate weapon skill. To guard a region prevents non-allied units from collecting taxes, gathering resources and recruiting peasants in that region (FROM THE FOLLOWING TURN ON) and non-allied units trying to travel through the region will be stopped with a certain probability."

This is definitely no bug.

Sorry, but I disagree about needing to issue a GUARD NOT. Where does it state this in the rules?

In your view of the rules a GUARD NOT is unnecessary.

Although it's not said directly, you can infer the fact that the guarding status remains active from the manual. "from the following turn on" is relatively clear I think Also the fact that there is a command "GUARD NOT" would, as mentioned, not make sense if the guarding was only for one turn, as well as the passage about losing the guard status when moving.

The rules could be more explicit about this. (e.g. adding a sentence like "Once a unit is ordered to GUARD a region you don't have to give that order again. The guarding status remains active until the unit is given the order to GUARD NOT or moved out of the region."). I had an ally once who interpreted the rules as you did and gave his guarding units a "@GUARD" order until I explained to him that it was not neccessary.

But trust me as a veteran eressea player: The rules are perhaps a bit ambigious here but the game behaves just like it should.

Rules were amended to say that the unit quits its guarding duties when it moves or is ordered to GUARD NOT. Like argelas said, not a bug, and now even more clear in the rules.

not a bug, then.