I personaly find myself busier in winter, 1. the days are shorter (obvious) 2. windows are generally dirtier quicker.
Go for 4 to 6 weekly cleans, at 8 weeks it's gonna take you forever to build a round up. maybe only offer 8 weeks for bigger jobs, say 30 quid+.
To be honest, winter or summer, it shouldn't make much difference to you building a round, although i wouldn't door-knock in the dark.
Print yourself some leaflets up, posting them in the dark will be ok.
Hope this helps.
tony