Once every 6 months should be sufficient. What I understood is that data corruption becomes a risk after 1 year without power. So you should be save with 6 months. You only need to power on the xbox, but I would advice you to also play games. It has no impact on the SSD, merely for your own fun
Currently XBOX uses UDMA2, this means that the data transfer speed is limited to 33 MB/s. The HW of the xbox does support up to UDMA5 (100MB/s) with the exception of the IDE cable. Recently a new BIOS was released unlocking UDMA5. You need an 80 wire IDE cable and a IDE-SATA adapter thats supports UDMA5. The startech adapter definitely does, other brands might support it but can also be UDMA3 or 4. At the moment both cerbios and evoX (titan) support udma 3 and up.
One note, although bandwidth increases by 3x, it doesn’t mean your loadtimes decrease by 3x. It seems to be fully game depended and different nr are going around. I at least noticed mafia loadtime to decrease with 50% (from 10 to 5 secs)