It's a gamble in the end. In recent years when I have crossed the atlantic on US carriers the SB sign was on for the entire trip. We had the odd little ripple here and there which couldn't even be described as light "turbulence". Now, let me be clear, I always keep my SB fastened, but on an 8.5 hour sector I'm going to need the loo. Could be just my luck that having kept my SB on up to that point the moment I release it, "bang wallop" 3 secs of significant turb and I'm kissing the ceiling! Not the crew's fault, not my fault., just an "act of God" as they say.
In contrast to the above experience with US carriers, I find that European carriers tend to be more realistic with the SB sign. The outcome is that when it is illuminated it will generally be respected. On my last eastbound oceanic flight we never saw the SB sign despite a number of encounters with, albeit light, turbulence.
I believe that the warning given by (most) crews in their speel to pax to keep their SB on when seated should be considered legal enough protection for them from being held responsible for any pax injuries sustained during unexpected turbulence.