In western cultures it is normal "etiquette" to say something like "excuse me" after sneezing. Some people believe that otheres in the vicinity should say "bless you" when you sneeze, although this is more superstition than etiquette.
If you are sneezing frequently this is going to become something of a chore. Personally, I would say that if it is common knowledge among your coworkers that you have allergies there is probably no need to apologise every time. In fact you could probably cover yourself for an entire day by saying something like:
Excuse me - my allergies are bad today.
If you said that the first time you sneezed then you have given an explanation for any subsequent sneezing.
Probably more important than verbal etiquette is that you follow good hygeine practice, such as covering your nose and mouth when you sneeze, and washing your hands (or using hand-sanitizer) after sneezing, even if you used a tissue or handkerchief, to prevent the spread of germs. If your colleages see you doing this then they will know you are showing them consideration and this really lessens any unspoken requirement to apologise for sneezing in the office.