I want to conduct an experiment. I want to observe time spent after a certain oscillation. I have actually did that and get some data but I want to know error margin.

I know the equation for osciallation time until rest. It should be T=2π√(L/g), but I don't know how to formulate for specific oscillation amount like time spent for 10 oscillation.

• $\text{time spent for 10 oscillations}=10\,\left(\text{time spent for 1 oscillation}\right)$ – AccidentalFourierTransform Dec 19 '16 at 19:26

If, as you correctly noted, the period of a pendulum is $$T=2\pi \sqrt{\frac{L}{g}}$$
As for an error calculation, there are a number of ways in which this might be approached. One simple way to do so is to perform the experiment a number of times. We can presume that the mean of these is the "true" value and that the error is $\pm \sigma$, where $\sigma$ is the standard deviation of the collected measurements.