StarSan will effectively last as long as it takes to use it - as long as the pH remains around 3 its active. Starsan doesnt like being made with hard water IIRC, so its easiest to pick up a 5l bottle of ashbeck water from tescos and just squirt 7.5ml of starsan concentrate into it - shake to mix - its done
Some people fill tubs with it and use it as a sanitising bath - from what i've heard they use it till stuff gets a "film" or it becomes visibly contaminated - again, as long as the pH is 3 or thereabouts, its working. Use the sprayer for instant sanitising on things like brew paddles, hydrometers, FVs before filling, etc - just spray till coated and leave.
Im a little fussy about sanitising and like spraying starsan on anything that stops moving for more than 3 seconds

for the lauter tun, mash tun and boiler it's not necessary as long as they are visibly clean - the boiling process is the most "sterilising" step of the whole beer process - so make sure you sanitise ANYTHING that comes into contact with post boil wort carefully (or everything you use if you make kits as there's no boiling step)
I know you said about pH testing, but for the sake of dipping a strip of paper in to it....

Primary 1: Nonthing
Primary 2 : Nothing
Primary 3 : None
Secondary 1 : Empty
Secondary 1 : None
DJ(1) : Nowt
DJ(2) : N'otin....
In the Keg : Nada
Conditioning : Nowt
In the bottle : Cinnamonator TC, Apple Boost Cider, Apple & Strawberry Cider
Planning : AG #5 - Galaxy Pale (re-brew) / #6 - Alco-Brau (Special Brew Clone) / #7 Something belgian...
Projects : Mini-brew (12l brew length kit) nearly ready
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