The dose depends on the amount of CBD contained within the carrier solution.
Ie: The CBD that I have used for my dogs came from an approved legal human dispensary in town, and I used the human weight scale to dose.
Mine contained 1000 mg of CBD per 30ml of solution
I have seen higher concentrations like 2000mg, 5000mg, and even higher per 30ml of solution. The really important thing is to get your CBD from a high quality source, with lab tests to prove what they claim. In Nevada USA, everything sold in a dispensary must be approved by licensed state labs..pesticides, PGRs, mold etc are tested for and the thresholds for contaminants are extremely low.
Initially I began with a smaller dose, and gradually increased it until the desired effects were reached, and then leveled off.
As @CachetheBC said, there does not appear to be any evidence that higher doses would cause an overdose (as long as there was less than .3% THC in the solution. I GUESS that if you gave too much, it MIGHT result in TEMPORARY Somnolence
Real CBD is just one of 500+ Cannabinoids that come from Cannabis Sativa plant. Always look for full spectrum products to take further advantage of the Entourage Effect created by other Cannabinoids. (enhanced effects) Breeders create multiple generations to breed plants that are high in CBD and low in THC..
ie: Charlotte's Web, named after Charlotte Figi (RIP), whose story helped make CBD what it is today.
It appears that there is little to no CBD in industrial hemp.
BTW,,There is no such thing as special pet CBD!
As with all things Cannabis: LESS IS MORE!