The cheeky answer would be "as many times as it takes". But I'm not cheeky today.
Five could be the magic number. For the purposes of this post five is, but take it as a guideline, not a rule. If it is a play I've designed before I skip straight to #2 or #3. After meeting with the design team and director reading the play with their ideas and vision in mind also allows things to come forward that you may have overlooked the first few reads. But, when it doubt read it again, with a pad of paper and pen by your side.
Yes, I have written about reading the script before in one of my Life Lessons from the Costume Shop and the number of times and order has changed since I've written that post. These are guidelines as I stated above. Read both, see which you prefer.
Once you've read your play, check out this article on places to do research other than the internet!