Security is Economics

How strong is strong enough?

You just need to make sure that your adversary would have to spend more to break your system than the benefit she would obtain by doing so.

But that's sometimes hard to measure, so we try to play it on the safe side.

Most symmetric systems have a key size measured in bits.
The more bits, the more possible keys there are.
But remember this is only an upper bound on the strength of the system!