Why not just have a test of it yourself?
Anyway there are two ways to use the GTS, each with their own benefits.
Firstly you could search for a Pokemon you want. It gives you a choice of up to 6, each with the pokemon they are looking for. If you have their required Pokemon on the right level then you can chose to trade with them and it happens instantaneously.
Or you put a Pokemon up you dont want/are offering and select which Pokemon you want, on a specific level. You then leave the GTS and play the waiting game until someone wants to meet your offer. It can take anything from 10 minutes to a few days, or never at all if you are asking for Palkia for your bidoof say.