The Ceres Liner bus from Dumaguete to Cebu is not that bad, I have been travelling this route several times now. It takes around 40 minutes from Dumaguete to the boat landing - around 60 minutes to cross over to Cebu Island and then another 3 hours up to Cebu City.
There are more non-aircon busses that ply the cebu to dumaguete route, but this is the schedule of the aircon busses as of today(might change so don't blame me)
Ceres Lines bus from Dumaguete to Cebu:
4.30am
6.00am
8.45am
12.50pm
2.30pm
Ceres Lines bus from Cebu to Dumaguete to Cebu:
6.00am
10.00am
1.00pm
3.00pm
7.00pm
The bus ride from Cebu to Dumaguete can be quite scary at night, so it might be safer to take one of the earlier trips
Happy travels