All of my communal area cleans are fortnightly and I do not have a problem with them being too dirty when I return. There have been no complaints so far. But saying that, it would be what they are used to. If they are currently receiving weekly cleans then that is probably what you want to price for. Two stairwells is pretty small, I do have one that is that size and I charge 35 per month to go there fortnightly. It is on the way to another fortnightly clean and I also have the grounds maintenance and windows on the property otherwise I probably would not have considered it. So in total I make around 100 per month on it but considering the amount of hours it takes, I can not complain. I hope this helps.
Cheers
Gilbert