My ex only rents a room, when she see's the kids she stay's at my place and I leave for the weekend, is that right, how long should this go on for...

I think that it is great that you are allowing her to have that time with your children. This is not only good for her, but great for the kids and actually fantastic for you to be able to catch a break as well. I'm always really impressed to see people working things out and doing it with all parties in mind. That is true unselflessness. To answer your question, I think that as long as she is unable to bring them to a place of her own and accommodate for them sufficiently, than it's okay. If it goes on too long( over 6 months to a year), maybe suggest that she arrange something different or compensate you in some manner.