The raids are in specific instances so you don't have to compete with other guilds. It's lock according to the raid ID which locks when one Boss is down.

How long are the raids? May take 30-60 min to get the first boss but the trash mobs respawn usually after an hour or more. Then the actual duration is probably 5-15 min. If there's wipes, then more boss attempts. Most guilds run 3-5 hrs. More than that, it gets tiring without a break.