This depends on if you are really walking the entire way or throwing in occasional running.
If you run a little, figure anywhere between 5 1/2 to 6 1/2 hours for a beginner. If you walk exclusively, expect 6 - 8 hours for a beginner. Note that you'd want to check the cut off times for races in big cities because they will usually open up the course for traffic in the 6 - 7 hour time frame. More experienced walkers can finish a marathon in the 5 1/2 to 6 hour range.
Anything faster than 5 1/2 hours would be considered competitive race walking speeds and is actually harder to maintain physically than running.
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it depends on how fast you walk. it could take three hours or five.
(apologies to whomever provided the above response -- that is, unfortunately, really inaccurate). 3 hours is a pretty good time for running a marathon. Walking a marathon takes at least 6 hours for a really fast fit walker -- generally walking a marathon takes 7 - 8 hours.
6-8 hours for most walkers (6 hours being a very fast walk, and 8 hours a more typical time). Very slow walkers could take 9-10 hours.
In US marathons held during 2012, the average finish time for men was 4:18. The average finish for women was 4:43.