The answer is based on several factors - the solubility of the BrdU, the size of your animals and the diluent- this is a good place to start - http://www.usp.br/bioterio/Artigos/Procedimentos%20experimentais/Administration_The_Laboratory_Rat-By_George_J_Krinke-2.pdf

They give MAXIMAL volumes, so dont think you need to go that high. The lower the better within reason as the volume has to be easy to taker into a syringe and give accurately.  I would recommend about 200 ul. Then you also dont need to have too many eppendorf tubes. Check the stability of the BrdU though - it might like to be at a higher concentration if you plan on using a stock over several days. 

Hello,

I used 100 µg BrdU per g body weight in rats and mice.

It was solved in the way, that the final concentration was 100 mg BrdU in 5 ml 0.9% NaCl-solution (first in less than 5 ml, because you need to add some NaOH-solution to get the BrdU diluted and then fill up to 5 ml).

Then you have to inject 0.5 ml of this solution per 100 g body weight (intraperintoneal).

It is well described in the Book "Adult Neurogenesis"  from Gerd Kempermann.

Sincerely Yours

Stefan