I'll suggest 4-5 nights in Tel Aviv, using some of the days to take day trips to Haifa (don't miss the Bahai gardens) and Acco, Nazareth and Safed and many more options. Then stay 2-3 nights in Jerusalem and explore the city (and take a day to explore the dead sea). In your spare time you might want to spend a night up north and/or stay in one of the resort/spa hotels.