Photoread as often as you want. If you're working towards having it flow in your life you will probably photoread 5 to 10 books a day and activate at least one per week, for a couple of months. (It increases your chances of spontaneous activation)
Photoreading a dictionary may help improve your vocabulary, if you work with a conscious intent to improve your vocabulary or even knowing the meaning then that will probably happen. Just remember the use it or lose it rule. Your conscious mind dumps currently useless information faster than you can say rip van wrinkle. If it doesn't fit with your day to day useage it's unlikely that you'll be a walking dictionary any time soon 
Alex
