Quite a bit of bickering in this thread. I think what's ideal and what was done 30 years ago isn't really relevant to this thread.
The problem is though, the question is not too different to how long is a piece of string. It varies dramatically based on the amount of hours per week you study/ can study, the school where you train at, the weather, your aptitude, plane/ instructor availability, the time between finishing one part and starting the next amongst a whole raft of other things.
The order modular students take courses are PPL, ATPL exams with night rating and hour building either during or just after, CPL-IR and finally MCC/ APS. Based on my personal experience and that of people I know, average times to do each part are:
1) PPL - 1.5 months to 6 months.
2) Night Rating - A few days and could be done during ATPL exams.
2) ATPL Exams - 6 months to 12 months.
3) Hour Building - You could do it along with the ATPL exams or you could go to Florida for 1 month. It's really up to you and there are pros and cons of both.
4) CPL/ IR/ MEP - 2.5 months to 5 months.
5) MCC/ APS - Less than a month.
There's also UPRT that needs to be done now. I haven't done it, but I think it's just a few days at most.
What you can see is it could be done in a year if you were very able and lucky; and I am sure there's someone on this forum who managed to do it in 11 months. That would be rather unusual though. Most people take 18-24 months. I wouldn't exactly call 18-24 months 'leisurely' as another posted did. There will be times are great intensity and stress in that time with less intense periods too.
Some might say it's impossible for someone to take 6 months to do a PPL if they are flying full-time unless they were useless, but I have seen it done. If you are studying in NW Europe in the winter, there could be week after week after week of bad weather. No sensible instructor would let a PPL student solo in marginal weather either - it needs to be good. I personally waited over four weeks between booking the skills test and actually doing it because the examiner was away on holiday for a week and two flights were cancelled due to poor weather. Stuff happens!
The other thing I'd say is that it isn't a race. It's much better to take 24 months and do it well than to rush and do it in 12 months. If you aren't ready to sit an ATPL exam, don't do it. For many airlines, your average score or number of first time passes is extremely important so a fail or scraping a pass isn't really ideal. Better to give yourself an extra month to ensure you are exam ready. If your ideal school has a waiting list of a few months and a school with a poor reputation which you visited and didn't like has a start date of tomorrow morning, it's probably better to wait for the other school.
Edit: I started typing this before spitfirejock posted and I didn't see it until after I had posted. What he says makes sense.