I don't own a Lumia 820 but this calculation shown be a good guide for how much time you can record.
According to the Lumia 820 specs on the Nokia website, the main camera records at
1080p with a frame rate of
30 fps. I assume these are the "standard settings" that you refer to.
Assumptions: compression ratio for the video recorded is
Frame size K = ( [Pixel Width x Pixel Height x Bit Depth] / 8 ) / 1024
K = ( [1920 x 1080 x 24] / 8 ) / 1024 = 6075 KB / frame
6075 KB/frame x 30 frames/sec = 182250 KB/sec
182250 KB/sec / 60 compression ratio = 3037.5 KB/s compressed
3037.5 KB/sec compressed * 60 s / min * 1 MB / 1024 KB = 177.98 MB / min
32 GB * (1024 MB / GB) / 177.98 MB / min = 184 minutes
So there you have it, approximately 184 minutes of HD video with 32 GB of storage.
Hope this helps.
Space saving tip: sync your videos to a PC whenever you have a chance.
Some of the calculation metrics were borrowed from a similar calculation made for iPhones.