 
 I ran into the same thing when my electric brush head started to flare after about two months. The sweet spot for me has been every 3 months, sooner if the bristles go splayed or after being sick. I tried stretching to four months and ended up with tender gums and a fuzzy feel on my teeth, so I cut back and set a 12 week reminder. Easing up on brushing pressure helped the heads last closer to that mark too.
For replacements I switched to Toothbrush heads and that made it easy to stick to the schedule. The 12 pack covered me for a full year and the heads fit my Oral-B Pro handle snugly. Only downside was the bristles felt a bit firm on the first couple uses and but they broke in quickly. Having a stack on hand meant I stopped putting off the swap when a head started to fray.
 
  
  
  
  
 