I purchase my olive oil and canola oil in large containers and transfer to smaller glass bottles for dispensing.

Oils begin to break down immediately after production, relatively unsaturated oils more quickly than saturated ones. Heat, light and oxygen all accelerate the process. As fats and oils degrade, they throw off free radicals which are known to cause health problems (specifically cancer). This is why we put such a high value on anti-oxidants (aka preservatives). The more care you give your oils, the less degraded they will become, even if they never reach the point where you can smell and taste the difference.

Depends on how quickly you go through the oil. I store my everyday oils in small translucent 99c plastic squeeze bottles, but I'll go through them within a month or two. Never noticed rancidity with this process.