http://ringofbrodgar.com/wiki/Claim_Shieldhttp://ringofbrodgar.com/wiki/CatapultIf you're using catapults, you only need to defend the machine 1 hr before it's ready to use. Each hour you can damage a claim shield by 5k hp, it takes roughly 16 hours to regenerate that damage. This means if you camp a base for 4 hours each day, it will take you 7.6 days to break through and gain access to destroying structures/walls (1 hour "wasted" on waiting for the catapult to dry). Alternatively, sieging straight it should take you about 13 hours to knock down a claim shield. Once the shield is down, vandalism is possible and siege machines can damage walls/structures.
TLDR it's possible, quality isn't really that important, but it's a no life only option. There's also some mechanics that can be abused to make things essentially impossible by large factions apparently, but I won't claim to understand it.