I would charge extra, every clean would be like a first clean, especially if it was near main roads/buliding work etc etc.
When you price this job why dont you give them some options
1 6 month clean
2 every other month clean
3 monthly clean
Adjust the prices accordingly 6 month clean being most expensive as more cleaning would be required, monthly clean cheaper but less work to do on your part, plus would seem cheaper as they are paying monthly rather than 6 monthly.
I've done this on a commercial before they wanted it weekly but i felt it didnt warrant weekly clean so i came up with every other week and it ended up i was better off £££ wise for work done
Andy