The single most useful thing to know is that a maxi skirt lives or dies by how much fabric swings free below the knee. Hold the hem and give it a proper shake in the shop. If it wraps itself round your ankles like cling film, you'll be fighting it on every pavement kerb for the rest of its life. A good one should move when you walk, not follow half a second later.
These earn their place from late spring through to the first properly cold snap, roughly April to October if you're anywhere near Norfolk's weather, and they're particularly good for anything where you're on your feet for hours without wanting tights. A wedding in a marquee, a long lunch that turns into a longer afternoon, a walk along the coast path where you want to look like you've made an effort but still need to climb a stile. Cotton and linen blends do the warm months, anything with a bit of viscose or a jersey content will see you through a mild autumn evening without you needing a coat specifically chosen to hide what's underneath.
How they age depends entirely on what's holding the waist up. A proper waistband, stitched down rather than just elasticated fabric folded over, will still sit flat in three years. The elastic only version will have gone slack by the second summer, usually right before you need it for something. Wash cold, hang rather than tumble, and check for a lining before you buy, because an unlined maxi in a pale colour will betray every single thing about your legs the moment you stand near a window. I bought one in cream linen a few summers back, lovely in the shop, absolutely see through the first time I stood near a patio door, and I've worn it twice since, both times under something longer.
They are the wrong choice for anything that involves stairs you can't see, bicycles, or a job where you're on your feet moving quickly all day. I wore one to a leaving do with a narrow staircase down to the cellar bar and spent the whole evening holding the front of it like I was presenting it to someone. They're also not kind to anyone shorter than five foot four unless the skirt is genuinely made for your height rather than just hemmed shorter, because a skirt cut for someone six inches taller sits wrong at the waist no matter how much you take off the bottom.
Try it on with the shoes you'd actually wear it with, not flat shop slippers, because two inches of heel changes where that hem lands by more than you'd expect.