The bus times, companies & departure locations seem to have changed, I took the bus to Ubinas from a stop at the intersection of Avenida Jésus & Cayro yesterday (February 4th 2017). The bus still leaves from Espinar & Sepulveda 3days a week I'm told, but at 6:30am. Best to ask around when you get to Arequipa. Also the name of the bus company I took was Coaquira Tours ph. 054-764979 or 95-8457746. The official tourist agency in the main square knew nothing of bus companies, departure locations or schedules, in fact they told me there WASN'T a bus at all!!
Also, it is easily possible to summit Ubinas the same day you catch the bus from Arequipa. Get out at Para, a junction in front of the volcano, not all the way down in the town of Ubinas. There are several possible routes up the volcano from here. You will probably have to camp overnight though, as there is only one bus per day in each direction - the bus back to Arequipa passes Para at around 9:30am.
I climbed via a ridge on the south side of the mountain (most direct route from Para, but steep, no trail & requires some scrambling). There was a huge flat area at 4,700m perfect for camping & a much smaller campsite for a couple of tents at 4,900m.