How long can I safely leave my Macbook Pro running?

I've set up my Macbook Pro 2011 to run a linux home server through virtualbox. I noticed that this laptop seems to get hot after a few hours compared to my previous laptop (toshiba, windows 8) which I could leave running for days and it never heated up. I am curious though how long I can safely run my Macbook without putting it to sleep? (I have NoSleep installed) I have no problems rebooting once a week or so, just want to know for certain if this actually necessary, cause otherwise I'd put a fan on it and stick it in a corner and forget about it. CPU usage seems to be around 10% when running the server from what Activity Monitor is telling me.

Laptops weren't developed with acting as servers in mind.

The overall-quality of the integrated components is lower than those of common servers. Additionally redundancy for important parts (like the cooling system etc.) is not given.

Especially consumer hard disks and the whole cooling system aren't designed to be run 24/7 a week and guarantee a long lifespan.

The in-case temperature will be generally higher than in a normal server.

This will effect particularly low-cost hard disks. The durability of hard disks may decrease with higher temperatures. (Opposite standpoint: Google)

Namely the Macbook Pro 2011 (i own one myself) heats up fast.

I always would advise to get a (cheap) MacMini, set it up headless and store in a vibration-free place.

If you insist on using your MBP as server, use something like SMC Fan Control (as mentioned/advised by yourself and dwightk , choose a vibration-free place and don't close the lid. Most laptops rely on losing some of their heat through the keyboard. Try to disable the discrete graphics card by turning off 2D/3D acceleration in your VirtualBox VM (please check this ticket).

It should be fine setting it and forgetting it. You might want to manually raise the minimum fan speed just a bit. It is usually running at about 2000rpm for noise reasons, but if you bump the minimum to 3000 it will keep the computer cooler. You can use SMC Fan Control or (I use) iStat Menus to adjust the minimum fan speed.