What could he/she do to the network? Can he move people's funds without their permisson? How long do a hacker need to control the network to make his changes irreversible?
What an attacker can do once the network is taken over is quite limited. Under no circumstances could an attacker take anybody else's money. An attacker's capabilities are limited to taking back their own money that they very recently spent, and preventing other people's transactions from receiving confirmations. Such an attack would be very costly in resources, and for such meager benefits there is little rational economic incentive to do such a thing.
Furthermore, this attack scenario would only be feasible for as long as it was actively underway. As soon as the attack stopped, the network would resume normal operation.
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http://en.bitcoin.it/wiki/Myths#Anyone_with_enough_computing_power_can_take_over_the_networkAs long as the attacker has 50%+1 of the hashing strength then it would be as if the attacker is the only miner mining. Immediately, the attacker can reject all or some transactions from being included in new blocks. With this power, the attacker could overtake the chain and rewrite previous blocks so as to reject a transaction of coins the attacker previously spent and then to double spend those coins in a new transaction to an address under the attacker's control.
