AbsoluteTiming

AbsoluteTiming[expr] evaluates expr, returning a list of the absolute number of seconds in real time that have elapsed, together with the result obtained.

Example

Measure execution time:

AbsoluteTiming[Sum[i^2, {i, 10^6}]]
(* {0.05, 333333833333500000} *)

Please visit the official Wolfram Language Reference for more details.