Of course they take longer! All my 4 weekly stay pretty clean for next visit where my 8 weekly get that sheen of dust and sapp, more spiders in corners/webs etc to the glass and frames meaning more scrubbing and rincing(i should price them more but i dont as a lot goto 8 weekly as they cant afford it 4 weekly i find).
I cant understand guys who do 12 weekly as they end up getting filthy! I have gained many customers in the past fir myself and others who say there window cleaner comes 4 times a year which isnt enough to keep them clean as always look filthy with bird marks, poo stains, dust, sapp, spiders poo, spiders webs etc etc.