How often does ptsd affect sleep?

Very often. Ptsd can cause nightmares and can be disruptive to sleep in other ways, like through depression. There are medications like Clonidine and prazosin that can help with nightmares, but psychotherapy also works. Try not to use alcohol to help you sleep as it can make the nightmares worse as the alcohol wears off, and lead to dependance. There are treatments for ptsd, seek help.
Extremely common. Sleep problems are so common among those who suffer from PTSD that it has been argued to not simply be a symptom of PTSD, but possibly, an independent disorder. It is safe to say that sleep problems are a hallmark of PTSD. Sleep medications can help as can CBT-I (Cognitive Behavior Therapy for Insomnia), especially if in the context of overall treatment for PTSD.
PTSD and Sleep. Ptsd can make it difficult to fall asleep or stay asleep, and can be associated with recurrent nightmares. These issues are very common, with some studies indicating 80% or more people with ptsd having their sleep disturbed in some way. Additional problems can develop also, such as alcohol and other substance abuse -- which can inadvertently perpetuate the sleep disturbance.
Symptom intensity. The more severe the comorbid symptoms of PTSD, the more sleep is generally effected.