Ferals only really need to shift to bear form at most twice.
Originally Posted by Ion
Once - in the first few seconds of the fight, shift to bear form to receive thrall's buff (halves all tank CD durations, doubles effective durations)
Second - at some point near or just after the 6th hour, shift to bear form again for 15 seconds, then activate glyphed Frenzied Regen (with the 4t13 bonus; without 4t13 there's very little reason to use FR anyway). They aren't doing this as a "I must survive Hour!" reason, but rather to lighten the load on healing output by providing a buff to healing received to every other raid member.
The glyphed version is safe for the feral to shift out of bear form any time after applying Mass Regeneration; the feral won't have the benefit, but the rest of the raid will.
That said, Feral itself can survive an Hour of Twilight in two ways, personally tested by me:
1) Obviously, Survival Instincts. 50% reduction is 50%. It's enough to safely survive, but should probably be combined with Barkskin anyway.
2) Frenzied Regen + Barkskin. Frenzied Regen increases max health of the druid, and barkskin can provide just enough protection to survive with a sliver of health. Technically, boomkin & resto's should be able to use this as well, but they probably won't have it keybound (or even aware it's possible). Note that you MUST be topped off with FR active WITH barkskin to survive, and when you do survive, you'll have somewhere between 1-5% HP left.
Also note: any feral should shift to bear form REGARDLESS to receive Thrall's buff. If only because then you have 20 second barkskin on a 30 sec cooldown, and don't need to worry about the 2nd hour you'll take (since without it, it's questionable if a 3min CD will be up).
youtube channel (watch for boss kills): http://www.youtube.com/user/fenriradra
last progression kill: yor'sahj (4/8 H-DS)