On copper pipes, real damage from acidic water typically starts within 6 to 18 months, depending on how low your pH is and how much water you use. Here is what moves that date for your house, the order the signs show up in, why the hot water side goes first, why it is never too late to stop it, and why switching to plastic PEX pipe does not end the corrosion.
A "pH water filter" is an acid neutralizer: a whole-house tank of calcite, which is crushed limestone, plumbed in where the water enters the house, that raises low pH well water to about 7. Google shows four things under that name, a pitcher, an under-sink cartridge, the whole-house tank and a chemical pump, and only the tank treats every pipe in the house. I go through which one you are looking at, why the limestone is the natural fix, how to find out whether your pH needs one, where the tank goes, and what changes after it is in.