[NoBullshit]

Since the voltage of a galvanic cell decreases over time, how does the concentration of the respective salt solutions alter the rate of voltage decrease? The cell in question is the typical Zn Cu voltaic cell.

Is the answer as simple as: If the concentration decreases then the rate of reaction decreases, which will mean a slower voltage decrease over time?

[Borek]

It is not a decrease "over time" but "over use".

Should be derivable from the Nernst equation. Ion consumption follows the Faraday's law.

[NoBullshit]

Ok, so if we assume the same conditions, if I decrease the concentration of my Zinc Sulphate solution what will happen to the rate at which the voltage decreases over use?
Sorry if this seems obvious to you but you did not answer my question.

[Borek]

I am not aware of any ready formula, but it is rather simple to derive from the Nernst equation with some assumptions. You need to take into account the load behavior though, as your question is rather ambiguous at this stage. Does the load draw constant current? constant power? variable current? If so, how does it change over time? Once that is known further calculations are a rather trivial physics, easy to combine with Faraday's law.

Sorry, I can point you in the right direction, but I can't do the work for you.