My rule of thumb is to cut runners once there's 3 or more plantlets, and they have developed enough roots to prevent floaters after planting.
Why 3 plantlets? Because those plantlets are soaking nutrients off the mother plant until that runner is cut, so 3-4 plantlets is about the point when the burden becomes too much...you usually see the mother plant start dropping leaves or looking a little pale.
Not a hard and fast rule by any means, but it's worked great for me.
Former advisor and planted tank geek...life's moved on though.