I've read that an 8gb card holds about 25 minutes of footage, but what about if I'm shooting at 720p60? Does the datarate change, or is it the same?

Thanks. :)

Card capacity depends directly on bitrate used, not fps. Therefore, for S&Q (overcranked) 60p it will indeed be lower, but for the "normal" speed 720/60p at 35 Mbps it's the same as for all other HQ modes.