One bottle can last over 6 months if you just brush with one drop morning and evening. There are 400 drops in a bottle; using 2 drops a day means a 200 day supply. Using this amount OraMD cost you less than $2.30 a month when buying the 3-pack.
Most people use 2-3 drops per brushing and use 1-2 drops in an ounce of water as a mouth wash twice a day which means you will go through a bottle in approximately 6 weeks. People with sensitive teeth and sensitive gums usually use more and will go through a bottle a month.