How long can i keep my laptop on (plugged in & charging) at a time without affecting its battery life?

You could leave the charger on forever and it won't affect the life of the battery. When the battery is fully charged the charger won't charge it anymore. I will no doubt get a lot of thumbs down one this but i know what i'm talking about.

The laptop will automaticaly stop charging it once its full, and it won't use the battery at all as long as the adapter is plugged in.

you can keep the battery plugged in and on mains power for as long as you like. The battery charging cuircuits in laptops have overcharging prevention circuitry. You might have problems due to heat buildup. A laptop cooling pad is always a good idea - http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=laptop+coolers

it won't effect your battery. if your laptop's battery is fully chargrd and still pluged in then it uses AC power only, not battery power.Some manufacturer suggest to take out the battery when you are using ur laptop on AC power for long time. it increase battery life.

Batteries are rated per-charge so if you never unplug it the battery should last longer if you do kill the battery run it all the way out before charging it up again even lithium can get a memory when they get older.

The charger stops charging when the battery charged completely. So don't worry about plugged in for a long time or forever.