When I first got my La Pavoni Europiccola last March (almost our anniversary!), I was living with my parents while waiting to hear back about grad school. They live in one of the better suburbs of Syracuse, and have pretty good water quality, largely due to local freshwater reservoirs. I think I descaled my Pavoni once in about six monts. Thankfully, the "it's a boiler with a spigot" design of this particular machine means that inspecting your boiler for scale is as easy as filling the machine is. You just need to shine a light in and check out the boiler walls. I first descaled when I saw a bit of a white film showing up on the inner walls, with some larger, crusty deposits of scale. A quick, 30-minute soak in a citric acid solution took care of it easily.
Now that I'm living in the city, though, I've noticed that scale builds up quite a bit more quickly. Our water here is pretty hard, and though I use a Brita filter to help cut down on some of that (I've actually read Brita is really more about chlorine and heavy metals, influencing dissolved minerals very little), I'm now on more of a 2-3 month descale cycle. Still, because the Pavoni makes it so easy to see the state of your boiler walls, I'm never surprised when I have to descale, and it doesn't feel like much of a chore because I do it before scale becomes a problem. Those folks with closed and concealed boilers, though, have to do a bit more guesswork, and thus I suggest adopting a regular descale as part of your maintenance schedule - unless your manufacturer prohibits it! Some do, for various reasons, so take note of your user's manual for descaling rules.
I personally like citric acid as my descale solution of choice. I like it because citric acid powder is cheap, easy to find in most grocery stores, and it works pretty well with negligible residue. White vinegar works too, but take my word for it, for every liter of vinegar descale solution you use, you'll need two liters of plain water to rinse away the taste and smell. Commercial descalers, of course, work fine as directed, but they're a bit more expensive. I like cheap things: I'm a college student.
My recipe is 2 tablespoons of citric acid powder per liter of you-could-brew-coffee-with-this hot water. Citric acid can be found in the canning or Kosher section of many grocery stores, sometimes labeled as Sour Salt, in the type of jar they sell other spices in. In my area, it's about $3 for one jar, good for maybe 10 descales. So, for my machine, all I have to do is empty the boiler, dissolve my citric acid into the hot water, and refill the boiler with the solution. I screw on my top cap, let it sit for 30 minutes or so, come back and check on the progress with a flashlight. Citric acid happens to turn my interior boiler walls a sort of salmon pink, which actually makes it incredibly easy to spot any remaining white scale. Again, concealed boilers don't afford the luxury of peeking in, so you just have to guesstimate. I'd say 30-60 minutes will be good enough for light to moderate scale buildup.
Once you're done soaking, empty the boiler (and reservoir if you've got one) of any descale solution, and flush flush flush. I find that about 1.5 liters of plain water gets rid of any sour tate in my machine. The key is to keep pulling samples to taste the water coming from your machine. Residual solution in the boiler or lines can affect the pH of the incoming fresh water, so tasting until you can't taste anything sour is a good method of judging when you don't need to flush anymore. Still, a very good rule of thumb is to expect the first shot or two to taste off. If they still taste off afterward, you need to flush more (yay!). This is why descaling is a weekend project to me - I don't want to wake up to a string of bad shots when I've got work in an hour. Taking care of it on Saturday ensures that the weekdays are greeted with tasty, syrupy shots.