You do it like this - bits per second equals bit depth multiplied by sample rate (24 * 44,100 = 1,058,400 bits/second) [if working with stereo, multiply this by two]. Divide this by eight to give bytes per second (1,058,400 / 8 = 132,300 bytes/second). Then divide by 1,024 to give kilobytes per second (132,300 / 1,024 = 129.199 KB/second). Then divide by 1,024 again to give megabytes per second (129.199 / 1,024 = 0.126 MB/second). Multiply by sixty to give megabytes per minute (0.126 * 60 = 7.57 MB/min)
You have seven tracks, so multiply by seven to give your total MB per minute (7.57 * 7 = 52.99 MB/minute). To convert from GB to MB you multiply by 1,024, so your 8GB card is 8,192 MB. Therefore it will hold 154.59 minutes (8,192 / 52.99 = 154.59 minutes).
I realise you got the answer above, but this shows how it's done so you can do similar in future.