No way. They are spongers, and brass necked ones at that. No thoughtful or caring person would dream of asking a casual friend to provide them with a home for six weeks, and, as you say, it could be longer.
They are looking for a way to save themselves money and live at your expense. If they come they will expect to be looked after and fed, with them making only token payments.
Just tell them firmly that you are not prepared to provide a home for anyone, other than immediate family for more than a weekend and you cannot possibly do it.
No excuses, no embarrassment, just be as upfront in saying no as they have been in asking you.