What sort of use does this car see? ( weekend / every day driver ? )
Are all that use this car attentive to its operation and will stop if the temps rise / odd noises ?
I tend to replace parts when they get near the end of their expected service life and before they fail in order to prevent unannounced breakdowns.
There are a two issues that come into play with the water pump : seal leakage and bearing failure.
My first pic is of a typical automotive water pump seal. There are a few other types typically found in non automotive applications and I can't be sure your pump has the type I'm referencing but the principals are all the same.
The retainer presses into the water pump housing like a cup type freeze plug. The spring loaded primary ring does not rotate and is sealed from leakage by a rubber bellows. The mating ring , seat cup and sleeve are attached to the rotating shaft or sometimes the water pump impeller.
The mating ring and primary ring are ground so smooth and flat that water can't get between them even under pressure.
Leaks can occur in two places, between the primary ring / mating ring. This is usually a small leak that could take a long time to get worse. If the rubber bellows fails, the leak is instant and major. In either case, stop leak wont fix this kind of failure.
Bearing failure can occur due to grease slowly drying out , getting past the bearing seal or due to water intrusion from a leaking seal. Signs of bearing failure can be noise or in extreme cases the fan going through the radiator.
A subjective test is to remove the belt and turn the water pump. There should be some drag during rotation ( this shows there is grease in the bearing ) and when when you start turning, there should be some amount of "stiction". Stiction is where starting friction is higher then running friction. shows that the sealing rings are being held together by the spring.
For more have a look here and do a net search for "water pump seal "
Water Pump Components | ASC Industries, Inc.