My prices have a 50% premium for a 12-weekly and a 100% premium for a 24-weekly.
Around a third take the 12-weekly option, which suits them and me. They pay less per annum and I get more per day's work, though obviously my round needs to be bigger. I think this is the key point. Charge enough and it's not a problem.
As to the extra workload, if a house is on a quiet back street, 12-weekly is perfectly fine and the glass looks good when I get back. The frames, however, do tend to show it - that's where the justification for the extra charge comes from.
I have been considering this, and I think the fact that I'm just inland from the coast might help keep them clean. Far enough that the air's not salt-laden, near enough that it hasn't picked up a lot of pollution. That might be utter codswallop, of course...
Vin