On average, each mss. takes approximately 6 weeks to read and consider, some take longer, some less time.
Reading the mss. is the least intensive step of the process. Once I've read your mss., I then take time to reflect on the story as a whole, and then upon each scene. Only after I'm comfortable with what I know about the story and how I would change it, do I begin making notes.
Sometimes I read a mss. twice to make sure I've understood what the writer is going for. Sometimes I break down the action chapter by chapter so that I can better help the writer construct a plausible plot.
Each mss. and writer requires my help in a different way. Some mss. have taken as long as 6 months to go through, while others I've finished in less than a week.