Translation:Sarah combs her hair for an hour.
Just to clarify --
"Sarah se peigne pendant une heure" = Sarah combs her hair for an hour (a habitual action, not something happening right now)
"Sarah se peigne depuis une heure" = Sarah has been combing her hair for an hour (happening right now)
Is that right? It's interesting that in English we distinguish these two ideas by changing the verb but use the same time word ("for"), whereas in French the same verb form is used but the time word changes.
I am French and I wouldn't say Je me peigne les cheveux. I would just say je me peigne or even commoner je me coiffe. But I wouldn't say je me brosse : here it is necessary to add les cheveux. So we say je me brosse les cheveux. I think it is because you can brush many things that belong to you: your shoes - your clothes.. but coiffer or peigner can only match with your hair. So for me it is redundant to say je me peigne les cheveux. Besides I am a woman and my hair is short so I never brush my hair : je ne me brosse jamais les cheveux.. je me coiffe or je me peigne. If my hair would be long I could use a brush and say je me brosse les cheveux. I think this has nothing to do with female or male but with the length of your hair and obviously the object you use to do you hair like in English . If you use a brush :une brosse then tu te brosses les cheveux..if you use a comb : un peigne then tu te peignes. The verb coiffer is more general it can be with a comb or a brush. I hope it is clearer for you now.
When to use les and when to use ses, that's the question! In a previous question it was "Il aime ses cheveux" and it was argued that it couldn't be "les" because that would mean he liked hair in general. Perhaps Sarah is a hairdresser and likes to comb anyone's hair? Or could it also be Sarah se peign ses cheveux? However it appears from other comments that "cheveux" is not necessary after se peigne! However she could be combing the dog or a horse - life is so complicated :-D
A possessive adjective is not needed or allowed for body parts when combined with a reflexive verb. The reflexive pronoun has already established ownership.
And you cannot "se peigner" a dog or a horse unless you are a dog or a horse. You can only "peigner" a dog or a horse.
Likewise, you cannot "se peigner" someone else's hair, you "peigner" their hair.