I'm currently doing a web design course with the Distance Learning Centre which suits me as it's all from home (they send you a CD to work from, or some courses are purely online I think). You do it at your own pace. You don't get a recognised qualification as such at the end, but you can submit your coursework to an examining body and apply for qualifications as you go along (costs about £100 a time I think). I don't use their email support service much but when I have they have got back to me fairly quickly (within a couple of hours usually).
I have found it a useful kick-start and it's nice to have the support available, but if you are super self-motivated and have the time you could probably learn a lot of the same stuff from books and free online resources, which would be cheaper.
Timescale - a lot depends on how much time you have available obviously! I've found it's the sort of thing you REALLY need to concentrate on, it's hard doing it with a toddler in the room or the telly on in the evenings. But I enjoy it, it's certainly bringing out my inner geek
Re qualifications - if you're applying for jobs it probably helps a lot to be qualified, but I'm going along Morningpaper's route of being self-employed and aiming to do basic sites for small clients, possibly with marketing consultancy as an adjunct to this (my background is in marketing and desktop publishing).
If you're self-employed then a portfolio and a good set of writing and selling skills would be fairly essential I guess, plus the requisite technical skills obviously. However I'm new to all this too so am reading the responses here with interest