In official competitions, there is an inspection time period given before the solve begins. Since the first world championships in 1982, this has been universally agreed upon as 15 seconds, this 15 second inspection time period applies to all WCA puzzles, with the exception being blindfolded solves where the inspection time (or memorization time) is included in the total solve time.

Speedcubers try to take as much advantage as possible with this inspection time as it allows the opportunity to plan out a significant portion of the solve for shorter events.

For the really fast events such as 2x2, pyraminx and skewb, the world's best speedcubers are literally planning out the entire solve in inspection, sometimes they are able to see multiple solutions and then choose which one they can execute fastest, I would highly recommend that all 2x2 solvers learn to solve the first layer in inspection as this is quite easy and allows you to solve the cube in around 3 seconds if you know CLL afterwards, if you know EG-1 as well then you don't even need to solve the full layer.

For 3x3, most speedcubers averaging around 10-20 seconds are able to plan out the cross in inspection, cubers averaging under 10 seconds are usually progressing towards planning out first F2L pair in inspection with the very best speedcubers in the world regularly planning out two pairs in inspection. Some elite speedcubers have tried to push this, however, planning further into the solve often requires less efficient solutions to allow the solver to look that far ahead

For bigger cubes, the inspection time still remains at 15 seconds, cubers often plan out bars of the centres, maybe first 2 centres for 4x4, first centre for 5x5, 2 bars of a centre for 6x6 and 7x7... Obviously as the cube gets bigger, the inspection time and apparent easiness of the scramble get less important.

In the competition, the judge will lift the cover off when you are ready, they will then start a stopwatch and call out 8 seconds and 12 seconds if you are still inspecting then, if you take more than 15 seconds to inspect then a 2 second time penalty will be applied to the solve and if you take more than 17 seconds then the solve will be a DNF (does not finish). Many speedcubers rarely use the full 15 seconds as it becomes risky and it can add to the pressure when you force yourself to plan too far into the solve.