Are you performing EIS on both the specimen and solution? Are you interested in both or either of them? If both of them, then I would say the old solution 

The use of Ringer's solution indicates that You are studying some biological samples - sudden changes in the conditions of the specimens can affect the results quite strongly.

On the other hand, the electrochemical disturbance related to the impedance measurement may be one such disturbance. Therefore, it would be best to investigate how much of an effect does the impedance measurement have on Your system.

This can be achieved, in its most basic form, by conditioning two specimens for some time, with one serving as the "blank sample". Measure the impedance of one specimen repeatedly (ten times the amount of measurements You are going to perform on a single specimen seems like a relatively safe estimate), while the blank is kept idle. Afterwards measure the impedance of both specimens and compare them, to see whether there are any noticeable differences. If none, the old solution can be safely used.

The solution remains the same throughout the experiment. For example, the system is maintained at the OCP and after 24 h you make the measurement.  In continuation the system remains on the OCP and after 150 hours from the start  the measurement repeat again and the same procedure after 300h.

Thank you so much my dear professors and colleagues  

Dear Yasser,

For prolong immersion study you have to use the same solution throughout the experiment. Then only you can get statistically acceptable comparative values.

Thank you Dr Karthik for your reply, and I think now I should learn some statistical specially ANOVA to analysis my data

I agree with keeping the same solution, but be aware of evaporation. You should keep constant the volume, refilling evaporated water.

EIS is done in an equilibrium state of a system. You must leave the specimen in the same solution for running impedance studies.

Thank you all

Gentleman

Again, my only suggestion is that you should never CHANGE the solution after reaching (1-3) day equilibrium state for running any corrosion EIS studies.

Only add small amounts of deionized water every week to keep the same level (marked preciously on the glass cell) of the fluid at the start of the experiment.

Enjoy your day

In my opinion you should keep the initial solution. Only two items you have to be aware of:

a) avoid evaporation of the solution (or arrange to replace the evaporated water)

b) maintain the reference electrode out of the solution during the rest periods.