Some commonly used abbreviations for the frequency of medications:
- SID, also s.i.d., is a veterinary abbreviation meaning to give medication once a day. Medical professionals for humans instead use the term qd (an abbreviation for quaque die). However, that term can be easily mistaken with the term qid (an abbreviation for quarter in die), which means four times a day.
- BID, also b.i.d. or BD, is a medical abbreviation for bis in die, meaning to give medication twice a day. This is the most commonly used pattern of insulin therapy for dogs.
- TID, also t.i.d., is a medical abbreviation for ter in die, meaning to give medication three times a day.
- QID also q.i.d, is a medical abbreviation which means to give the medication four times a day.
- PRN is a medical abbreviation for pro re nata, meaning to give medication as the situation demands or according to need.
- Qxh or Qxd once every x hours or once every x days.
- More abbreviations here. 
- Notes at the link above offer a good formula for calculating changing the number of times a day dosing. Changing a TID dosage to BID means multiplying a single dose times 3 to get the total daily dose. This number can then be divided by 2 for BID dosing. This can be used for determining any type of dosing pattern.