For example the wards erected around Callandor were tied off, inverted and made to last forever, until the right person came and undid them. There is no indication that any of them lost effectiveness over time. As RJ said, a weave that is tied off could last forever - it is up to the intentions of the channeler to determine whether tying one off is appropriate or not.

A weave isn't a physical construct; the flows form an energy matrix, and this matrix produces an effect, but it is in no way bound by the rules of the physical world. This is part of the reason why Aes Sedai are considered so much more powerful than a blademaster despite the fact that the Tower had not participated in large scale battles in centuries. A single Sister, not even among the strongest, could weave a barrier that could hold out any number of enemies indefinitely. You can't "beat down" a weave creating a wall of air - you'd need another channeler to neutralize it. A fireball generated with the Power, for example, can't be deflected with a Shield - we are told that any fire created with the Power is so intense that it burns through anything it is meant to burn through or touches. You can, if you are a channeler and know how, slice through the weave - in other words, disrupt the energy matrix, but for the most part a fire woven from Fire would burn through anything it passed through without losing energy or strength (for example Rand's massive wall of fame in EotW stopped the advance of the Shadowspawn for the most part; they would need to find a way around it to progress). It is this power that made the Aes Sedai adopt the Three Oaths - if the Aes Sedai of the White Tower wanted to dominate the Westlands by force, they'd easily be in a position to do so. All that would be needed would be an army of loyal warriors and bodyguards. The only thing preventing this from happening in the Seanchan Empire was their lack of unity.