how long to foreign visitors stay? Do I have to

You greet them at the door and invite them inside. You do not have to ask them to spend they night. I always make sure that I have plenty of leftovers in the fridge for them to eat. Have a guest room with a bed set up for them. My guests usually stay for around four to five days. If you go out on the town with them they don't come back..at least mine haven't so I don't do that anymore. I would imagine that if their needs got too low they would leave, so make sure they have things to keep them entertained and such, if your Sim leaves for work.