I have reused the same staining solution for approximately 50 big gels(18cmx20cm). After your 2nd dimensional run is over, try to wash the gel in deionized water for 2 times, then fix it in 10%acetic acid :50%methanol:water solution for 20-30 minutes. After that transfer the gels for staining.Keep it in staining solution for 3-5hrs/overnight.