Hello and welcome to Just Answer. I am a licensed veterinarian and would be happy to answer your questions.
There is a large range of dosing of this medication, but in general, it is usually given about 30 minutes before the event and will last anywhere form 6-8 hours.
Ask your veterinarian about a safe dosing range and if 1/2 tab is not enough, they will be able to advise how high a dose can be given.
Hope that helps. Please let me know if you have any further questions.