That’s difficult to say. As everyone I followed the instructions from the textbooks: “replace your dehydration fluids after two or three samples”. But I also noticed that those few samples dehydrated in an already several times used dehydration series turned out okay. That kept me thinking on economizing on alcohol.
My girlfriend did some approximate calculations. She assumed that each jar in the dehydration series contains 99ml, 49ml or 24ml of dehydration fluid respectively and that 1ml of water per sample was brought in the first dehydration step (ethyl alcohol 70%).
I assumed equal concentrations in both the isopropyl alcohol and in the sample at the end point of each dehydration step.
These calculations show that the final water content in the isopropyl alcohol III stays very low even after dehydration of lots of samples.
Given the fact that some water content in the dehydrated sample can be tolerated, it’s not at all necessary to refresh the alcohols that often.
I consider a final water content in the specimen of less than 1% prior to infiltration with paraffin wax satisfactory.
Dieter Gerlach even suggests in “Botanische Mikrotechnik” that some samples benefit from only partial dehydration, up to ethyl alcohol 90%.
The graph shows the results of the calculations Annick made: even in the situation of 24ml alcohol + 1ml of water from the sample (“24 + 1”), 24 samples can be dehydrated in the dehydration series before the last alcohol in the series drops below 99% alcohol concentration.

Of course, the water content of the final alcohol step isn’t the only consideration: samples brought in a (too) low alcohol concentration tend to macerate; thus giving poor sections/slides when these stay there too long. A considerable drop in alcohol concentration occurs especially in the first dehydration step (ethyl alcohol 70%). And there is also the contamination of the alcohols with rest products from the fixation, fats and oils from the sample, chlorophyll… to consider.
It seems safe to say that the final amount of water in the dehydrated sample should be less than 1% and the alcohol concentration of the dehydration fluids shouldn’t drop more than 10% below their original concentration at any stage of the dehydration process.
That gives the following results regarding the amount of samples to dehydrate per dehydration series:
Scenario 
# samples 
24 ETOH + 1 H_{2}O 
3 
49 ETOH + 1 H_{2}O 
7 
99 ETOH + 1 H_{2}O 
15 
Given all the assumptions in the calculations a safety margin is appropriate.
% IPA after dehydratation of Sample, IPA III init.100 %

99+1 
49+1 
24+1 
Sample 1 
100,00% 
100,00% 
100,00% 
Sample 2 
100,00% 
100,00% 
100,00% 
Sample 3 
100,00% 
100,00% 
99,99% 
Sample 4 
100,00% 
100,00% 
99,99% 
Sample 5 
100,00% 
100,00% 
99,98% 
Sample 6 
100,00% 
100,00% 
99,97% 
Sample 7 
100,00% 
99,99% 
99,95% 
Sample 8 
100,00% 
99,99% 
99,94% 
Sample 9 
100,00% 
99,99% 
99,91% 
Sample 10 
100,00% 
99,98% 
99,89% 
Sample 11 
100,00% 
99,98% 
99,86% 
Sample 12 
100,00% 
99,98% 
99,82% 
Sample 13 
100,00% 
99,97% 
99,79% 
Sample 14 
99,99% 
99,96% 
99,74% 
Sample 15 
99,99% 
99,96% 
99,69% 
Sample 16 
99,99% 
99,95% 
99,64% 
Sample 17 
99,99% 
99,94% 
99,58% 
Sample 18 
99,99% 
99,93% 
99,51% 
Sample 19 
99,99% 
99,92% 
99,44% 
Sample 20 
99,99% 
99,90% 
99,37% 
Sample 21 
99,98% 
99,89% 
99,29% 
Sample 22 
99,98% 
99,88% 
99,20% 
Sample 23 
99,98% 
99,86% 
99,11% 
Sample 24 
99,98% 
99,85% 
99,01% 
Sample 25 
99,98% 
99,83% 
98,90% 
Sample 26 
99,97% 
99,81% 
98,79% 
Sample 27 
99,97% 
99,79% 
98,67% 
Sample 28 
99,97% 
99,77% 
98,54% 
Sample 29 
99,96% 
99,75% 
98,40% 
Sample 30 
99,96% 
99,73% 
98,26% 
Sample 31 
99,96% 
99,70% 
98,12% 
Sample 32 
99,95% 
99,68% 
97,96% 
Sample 33 
99,95% 
99,65% 
97,80% 
Sample 34 
99,94% 
99,62% 
97,62% 
Sample 35 
99,94% 
99,59% 
97,44% 
Sample 36 
99,93% 
99,56% 
97,26% 
Sample 37 
99,93% 
99,53% 
97,06% 
Sample 38 
99,92% 
99,50% 
96,86% 
Sample 39 
99,92% 
99,47% 
96,64% 
Sample 40 
99,91% 
99,43% 
96,42% 
Sample 41 
99,91% 
99,39% 
96,19% 
Sample 42 
99,90% 
99,35% 
95,95% 
Sample 43 
99,89% 
99,31% 
95,70% 
Sample 44 
99,89% 
99,27% 
95,45% 
Sample 45 
99,88% 
99,23% 
95,18% 
Sample 46 
99,87% 
99,19% 
94,90% 
Sample 47 
99,86% 
99,14% 
94,62% 
Sample 48 
99,86% 
99,09% 
94,32% 
Sample 49 
99,85% 
99,04% 
94,02% 
Sample 50 
99,84% 
98,99% 
93,70% 
Sample 51 
99,83% 
98,94% 
93,38% 
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