Quite common in my company. If your roster has you flying 2 or 4 sectors then standby for a few hours it's perfectly legal as long as you are within your FDP. Also, just because you have flown your two sectors or whatever does not mean you are off duty, your duty is what you are rostered for.
If on the other hand, you flew a rostered duty and on completion were told to go on standby I think you may be able to refuse if you so chose, but again I don't think this would be illegal.(Assuming any potential callout flight could still be completed within FDP)