I've been growing echeveria for about half a year now, it sits on a south-west facing window. Recently I noticed that the lower leaves started turning yellow and getting kind of wilted, though the top is still green. I water it roughly every two weeks. Is this normal or am I doing something wrong?
You're most likely overwatering the plant - soft yellow leaves at the bottom are a classic sign of root rot. Echeveria is a succulent, it needs much less water than it seems, especially if the soil doesn't drain well and the pot has no drainage holes. Try watering less frequently (once every 3-4 weeks in summer, even less in winter) and only when the soil is completely dry, plus make sure water doesn't sit in the pot. If the leaves have gotten really mushy and the rot has spread to the middle of the rosette, you'll need to take the plant out and check the roots - trim off the diseased ones and repot it in fresh soil.
Try letting the soil dry out completely between waterings and repot the plant into a looser substrate (echeveria loves a mix with perlite or coarse sand). The soft lower leaves do suggest too much moisture, but that doesn't necessarily mean the roots are completely rotted - often it's just a matter of rethinking your watering schedule and improving drainage. Your window gets more than enough light, so the problem is almost certainly water, not lighting. After repotting, give the plant a couple of weeks to rest without watering, and it should bounce back.
Actually, it's not always overwatering - I had the same thing happen, and it turned out cold air at night by the window and sharp temperature swings were to blame. Echeveria suffers less from water than from stress: I noticed soft leaves on the bottom right after we started opening the window for ventilation on cool days. I moved the plant away from drafts and started watering less often (once every two or three weeks), and the leaves stopped wilting. Try moving your echeveria away from the edge of the windowsill where it's colder at night, and see if it improves over a week or two.
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