I see that the maximum size SD card that is currently supported is 256GB. Can anyone tell me approximately how many minutes I'd get, assuming I'm at 24/48 and recording 24 tracks? Thanks.

That would be 24 x 48k = 0,139 MB/sec (bitrate is 48,000 x 24, /1024/1024 for Mbit/8 for MB/sec) or 8,24 MB/min per track, so x 24 tracks is 198 MB/min. 256 GB will then give you at least 18 hours.