Six months is probably too long.
Consider this... pointing a fan at the insides of a desktop computer with all of the other fans turned off is a bad, bad idea. The fan in your laptop, just like the fan in a graphics card or the one on a desktop CPU, is designed to push air through a certain area to cool the system. Notebook coolers are generally pretty weak and only push air around underneath the laptop. Even if you sat the laptop on top of a box fan, I doubt it would be able to effectively channel air through the heatsinks inside.
You should try to figure out some way to push air through the laptop, or just take the laptop apart and replace the fan... shouldn't be too expensive. Or stop using it before everything gets fried.