Your vet should tell you exactly how long your dog should fast before coming in for blood work.
But most times a dog needs to fast a minimum of 12 hours before going to the vet for blood work so that the results will be accurate.
You can fast your dog a bit longer than 12 hours but usually 12 hours of fasting before the dogs blood work is enough in most cases.
Also if the vet has you give the dog any medication prior to the blood work you may need to change the time you give your dog the regular dose of medication.
But as long as you're fasting your dog for at least 12 hours before the blood work everything should be fine as that is what my vet told me to do before I took my dog in for blood work.
The vet said to fast the dog 12 hours before the blood work and everything came out okay on the blood work.