Yes, Benadryl (diphenhydramine) can safely be taken more than once a day for most individuals. In fact, the product labeling for Benadryl states that it can be dosed every four to six hours, meaning that you can take up to 6 doses per day.
For adults, you can take one to two tablets (25mg to 50mg) by mouth every four to six hours. This means you can take up to 12 tablets per day (6 doses of 2 tablets).
The directions for Benadryl line up with the typical duration of action for the drug. For most, each dose will last four to six hours. However, it is important to note that this can be prolonged in certain populations, such as in the elderly or in those with liver impairment.
Lastly, if you are taking Benadryl to prevent allergy symptoms, it may be worthwhile to look into "second-generation" antihistamines as they last longer (up to 24 hours) and are far less sedating than Benadryl. Another alternative could also be intranasal corticosteroids, such as Flonase or Nasonex. They are very effective for seasonal allergies and should not cause sedation.