When the restaurant is crowded - we are very busy. If you want to say that there are no tables available, you can say 'we are fully booked' When you seat the customers, you can tell them something like "Please be aware that tables are available only for a two hour period, so that we can also seat other people who would like to dine with us". If that sounds too formal, please let me know. I am not sure what kind of restaurant you work in.