UK ladies please.... living together rules...

Hi

I need some advice please. My OH and I hope to move in together eventually. Does anyone know the rules on living together and benefits such as housing benefit? I've heard he can only visit and stay so many days too before I have to declare him living with us. I'm a single mum and been seriously ill for a few years so I need to make sure I do this properly.

Thanks in advance for any help x