when do you replace a fuel pump?

When it stop delivering fuel to the fuel rail. They usually die and leave you stranded.

If it's working find leave it alone until it stops working, fuel pumps are not fun to work with.

You need a set of pincher clamps, like a pliers what has a locker on it, to hold just enough to pinch the fuel line. Could never have replaced a Porsche's fuel filter without them.

usually they will emit a high pitched steady whine when they are getting close to failure time- do it before it fails if you can.