It depends on blood collection time, collection frequency,blood sampling site and method. Based on an average rat body weight of 200g, 5-8 mL blood can be collected through orbital cavity.A typical volume of 0.3-0.6 mL is separately drawn at different time points for pharmacokinetics studies. For continue sampling of a considerable amount of blood, physiological saline supplementation is often required during the collection.
I believe 2.5 to 3.2 mL in a 0.25 kg rat is safe if you will not use the animal after the collection.
A safe maximum for a single sample is 1.25% of body weight (1.25 ml/100 grams of body weight) taken every 2 weeks. Animals on chronic studies requiring multiple blood samples may require hematocrit monitoring.
As mentioned by Aloksir, around 1.25 - 1.5% of body weight blood can be taken safely, so normally 2.5 - 3 ml of blood can be taken safely (Normal expected weight of Rat = 200 - 250 gm).
One more thing, for single time blood collection - retro-orbital plexus method is advisable while for multiple time blood collection - Tail vein method is more suitable!!!
For best guide and complete explanation have a look to the attached file.
Most of the answers are almost correct but you have to consider weight of animal, age and route of blood taken (i.v. from tail or directly from the rat heart) and is it single taken or repeat taken. However, about 2 ml is acceptable.
It is approximate 1 to 1.5% of body weight. This range is very safe.
Below are some guidelines from NIH.
http://oacu.od.nih.gov/ARAC/documents/Rodent_Bleeding.pdf
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