How long should you wait to drink alcohol after taking allergy pills?

Never. Most over-the counter allergy pills can cause some amount of sedation. Even "non-sedating antihistamines " such as Claritin (loratadine) or Allegra make some people drowsy. Since both alcohol and antihistamines can impair mental funtioning and reactions times, it is never advisable to mix these medications.
Depends on pill. In general shouldn't drink at all that day but really depends on which type of pill. Often the label will tell you.