Use ffmpeg:
ffmpeg -i filename.flv
Look for:
Input #0, flv, from 'The Veronicas - Hook Me Up.flv': Metadata: starttime : 0 totalduration : 179 totaldatarate : 841 bytelength : 18910448 canseekontime : true purl : pmsg : Duration: 00:02:59.39, start: 0.000000, bitrate: 843 kb/s Stream #0:0: Video: h264 (Main), yuv420p, 640x480 [SAR 1:1 DAR 4:3], 726 kb/s, 23.98 tbr, 1k tbn, 47.95 tbc Stream #0:1: Audio: aac, 44100 Hz, stereo, fltp, 128 kb/s