As ever it depends on the technique being used, nature of your fusion, and the number of levels done.
Often surgery in the lower back is done in two stages. Often the first stage is through the front of your abdomen, and the second through your lower back. These operations are normally done one or two weeks apart.
Each operation will keep you in hospital on average a couple of nights. However, some patients do go home after one night after the first procedure. If you are only having a one stage operation you are often in hospital 2-3 nights.
Make sure to check with your Consultant which type of operation you are having.