I have many in-ground largish bogs and have not in fifteen years had to have a wholesale change of the medium. What you do find is that initially, the medium 'settles' by a couple of inches as it compresses, then in subsequent years (4-5 years onward) as the medium loses its acidity, that's when it starts to break down, starts to slump, and release nutrients into the soil, obviously not a desirable state of affairs. By this time anyway, the soil has settled to a point where it isn't a very open structure anymore and lowers the ability for oxygen to get to the roots and your plants have grown to a point where they need splitting to create space.
What I do is to dig up the plants in spring, split them and replace them, but at the same time I 'refresh' the medium around the plant by turning it over to open the structure to let in oxygen, mix in a bit of sulphur to re-acidify the soil to stop nutrient release and dig in pine bark and shavings, again to open the structure add long term acidity and add some tannins. Be careful with the sulphur, it doesn't work until the soil is over 10C, and don't get any concentrations near the plant rhizome of roots as when it starts to work the acidity local to the sulphur is sufficient to burn the plants, and kill them.
Once that's done you're good till the plants need splitting again in 4-5 years time.
Something that helps enormously is teh growth of Sphagnum on your bogs it tends to assist in keeping the soil acidic. My bogs are/were covered in it and it looked good as well as performing a useful purpose, however, I now have the dreaded star moss that is taking over the bogs, crowding out and reducing the Sphagnum moss growth. I hate that stuff.
It's worth investing in a good PH meter, this is the one I have used for 15 years or more, its more expensive than a lot, but it is very good
https://www.amazon.co.uk/Accuracy-Hygro ... meter+VT05