They generally last 4 weeks, if you get one you have to tell them if it's gonna be any longer, but should really be no longer than 6.
I did one in a research lab at Manchester University last summer, found it really worthwile and gave me a lot of skills that I otherwise wouldn't have gained without it.
I did mine in physics, though turned out to be more technological than anything. I found it looked really good on my personal statement too