Also, you have omitted a key piece of information when outlining your experiment: Are you using sugar cubes or are you using granulated forms of sugar that come in one-kilo bags? It's quite fair to say that the dissolution time will vary considerably considering how the molecules are clustered.
Indeed, if you haven't really been too organised when conducting the experiment, you should think about your lack of control with the variables could influence the result. For example, if you are dissolving salt in hot water but sugar in cold water, it may give you an inaccurate result that will make it quite difficult to compare the two with ease.
For more advice, visit specialist science websites likewww.infoplease.com/cig/science-fair-projects/salt-sugar-diss or talk to your science tutor for more information.