The reaction you are talking about is the decomposition of sodium thiosulfate with HCl. The equation is written below.
Na2S2O3 + 2HCl --> 2NaCl + S + SO2 + H2O
The sodium thiosulfate decomposes into sulfur, sulfur dioxide, and water. The rate of a chemical reaction is measured as the change in concentration of the reactants or products over time. The general rate equation for this reaction is:
rate = k[Na2S2O3][HCl]^2
This equation measures the rate as a function of the concentration of the reactants. Note that the concentration of HCl is squared due to its coefficient in the chemical reaction. Also, the k in the above equation is called the rate constant. It sounds like we don't really have to worry about it in this case.
It sounds like the X and the concentrations you are talking about are the concentrations of sodium thiosulfate in the various experiments. The volume of water or the reagents don't matter, only the concentrations. The HCl sounds like it stays the same, and the rate constant k certainly stays the same, so you are basically left with the rate of the reaction being dependent with the concentration of Na2S2O3. We would say that they are directly proportional. It sounds like this is the ultimate outcome of your experiment since you said that as the concentration of the solution increases, the time for X to disappear decreases. In other words, as the concentration of the Na2S2O3 increases, the rate increases. The more the rate increases, the less time it takes for the reactant to disappear.