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Not a question that can be easily answered. This depends on your network speed/abilities, bitrate of the media in question, whether the media requires transcoding, and network overhead (i.e., normal usage of your network for other devices/activities).

Theoretically? The limit is going to be your ISP's upload speed limit for you. Take maybe 80% of your upload and divide by the bitrate of your media. This is under ideal circumstances. Even more theoretically, there could be a disk I/O limit you'd hit at some point (this would require you to have a 6+ Gbps upload speed capable network and hardware...meaning it's not going to be the bottleneck).