I have a MacBook Air (11-inch, 2011) ,I am now downloading iTuneU courses 24 hours.There is only one app iTunes running,is it ok?
One more thing I really want to mention. I would not recommend to use any of those MacBook body cases, such as this one for example
Basically, the cooling of the MacBooks works through a gap below the screen where the hinges are, the aluminum body, and the keyboard. If you cover you MacBook in plastic, it will reduce the heat escape through the body. Also, I often see people with key board covers. I think this is a really stupid idea, because it prevents air circulation in the MacBook and causes the heat bottling up inside.
However, I have to say that at home, even if I have my MacBook closed and connected to a external monitor, I never had any issues when it ran for a couple of days.
Also, what I noticed when I did heavy stuff on my MacBook, is that the vans will produce some extra noise that. You can take this as a warning signal. However, the MacBook would also turn off itself automatically, if it gets too hot, to protect itself from heat damage.
To keep it short: I wouldn't worry about letting your MacBook running for a week or so to just download stuff, that's just fine.