This answer shows that the mass of the ISS has changed by about 27,000 kg in the last three years.
09-Mar-2011 417,289 kg (per @DavidMorris' comment) 10-Jun-2015 390,377.5 kg 22-Jun-2018 417,501.6 kg
I suppose it will fluctuate a lot depending on the number of capsules docked, plus the current level of supply water, food, experiments, and new modules.
Is there a plot of the approximate total mass over time, say a decade or longer? Perhaps a fancy one that shows steps indicating docking and undocking of the other spacecraft? Something like this https://i.stack.imgur.com/t0Pda.png except for mass rather than altitude, but also the overall growth as well?
update: with the addition of the 2011 value and it being so close to the June 2018 value, it looks more like fluctuation rather than a steady trend upwards.