I have been studying via DL for three months (evenings and weekends) and am about half-way through Mod 1.
To come back to Bandit 650's original question I would say that there are at least three factors that affect the time spent on a DL course:
1. Each person has their own optimum pace for learning (how quickly information is assimilated, and how well it sticks). For me it is slow and steady, but others may race through and it all goes in. Good for them!
2. Each person has their own circumstances (job, age, available time off, kids, finances), and although getting through the course is a high priority, it may be one among many, and these may have to be weighed carefully. I'd rather take a couple of extra months to complete if my family is going to be intact at the end of it!
3. Each person has a different level of existing knowledge, and so there may be elements of the course that are easy for some (merely revision) and very difficult for others (completely uncharted territory). For example I have a background in communications technology and so Maths and Radio theory was mostly revision. However Meteorology is the bane of my life.
It is true that the longer you take, the more opportunity there is for things studied a while ago to be forgotten. Conversely the quicker you rush through some topics, the less opportunity there is to really consolidate your knowledge.
Like most things it is a question of balance.