I beleive that they reccommend that you apply for a renewal of an EAD 90 days before the current EAD is due to expire. If I'm right about that then I'd assume that a renewal could take just as long as a regular EAD application.
However, I beleive that the advantage of renewing an EAD is that as long as you can show proof that you have applied for a renewal, that is considered proof of your eligibility to work so you are able to work while you wait for the renewed EAD to arrive.
Mark