I think this is the standard DSM IV definition:

Types of PTSD
There are three types of PTSD: acute, chronic, and delayed onset. In acute PTSD, symptoms last less than 3 months. In chronic PTSD, symptoms last 3 months or more. In delayed onset PTSD, symptoms first appear at least 6 months after the traumatic event.

I think I do know someone who had acute though, but she had symptoms for years, maybe she just didn't fit the definition of PTSD specifically beyond 3 months. Who knows? I don't think these categories mean that much, real life tends to be less easily categorised.