Do these vary parish to parish?
Yes. For most of the week it is just canonical hours that replace the normal Agpeya. There is a 1st, 3rd, 6th, 9th, 11th, and sometimes 12th hour for the morning (day) and for the evening (overnight), so they can be grouped together in different ways. Some pray all of the day-time hours around noon, some split it into two services, in the morning and at noon, or later. There is also variety on total length, depending on how many commentaries & long tunes are used. Great Friday is pretty much all day, but will vary between 7-12 hours in length from church to church. Thursday's Liturgy should finish by 12:30, since the canon's require that on a feast day the fast be broken by noon (so Communion by noon then a little more time to wrap up), but most ignore this rule and go till 2 or 3.
