Is the taking city part of cyrodill going to take place once a week? every few hours? once a month? or anytime like GW2?

Months.

http://help.elderscrollsonline.com/app/answers/detail/a_id/1396/kw/cyrodiil

24/7/365

Campaigns reset once every few months, but you can jump in and out of the campaign any time you want. In other words nothing like GW2.

If you want to see the model then go get a DaoC trial and try it out. Only difference is that in DaoC the campaign never resets

Not really anything like GW2. taking the capitol city will take months, It was also said that when the campaign ends and starts over the keeps would not reset as they want to keep player accomplishments meaningful and they dont want people just waiting for stuff to reset. So if your guild goes into one of teh enemies homelands in Cyrodiil and takes there keep and guards, and upgrades it. the only way to lose it would be for enemy players to take action and take it back. It looks like they are going for the true Darkage of Camelot system and not what GW2 did.

By jump in and out i hope you mean enter and leave Cyrodiil. But they said you would always be in the same campaign as your guild and see teh same people there. (kinda like a server) they said if you swiched guilds you would have to wait for the current campaign to end before being put in the new one. they said there will be other options in these cases like spending $ but they do not want people campaign hopping to be with a winning side.