How long can I safely leave my Macbook Pro running?
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).