We did not see any significant effect to the result within five times. If the electric current exceeds 75 mA at 130 V, it is necessary to change the buffer.

You can use your buffer several times as long as you don't see obvious contamination it has no effect on your gel results.

I reuse it quite a bit; two/three times is fine. Just make sure you recirculate the buffer, or else most EtBr will migrate towards the cathode and you will get unevenly stained gels or worse, a drop in sensitivity. Also, make sure the gels are prepared with the same batch of electrophoresis buffer.

Hope it helps!