It would last as long as the applied force continued, or until the accelerating object hit something.
The weight should be shared between the two string equally. Therefore, tension in each string, T is;
T = 120 N/2 = 60 N
Efficiency is calculated through dividing the actual mechanical advantage by the hypothetical mechanical advantage:
- the actual mechanical advantage is 9J because that's how much work the light bulb doing
- the hypo. mechanical advantage is 100J. Ideally, in a perfect world, the light bulb can convert 100J input into 100J output, but do to resistance and other factors it is not possible.

change the decimal to a percentage:

the light bulb had 9% efficiency