I've been to El Salvador for 3 weeks last August and September. I rented a car in San Salvador en traveled around the country en spend a most of the days in El Zonte. I never felt unsave but there are some spots, particularly in San Salvador where you shouldn't go to (and probably will not go to by accident). The country is very beautiful and I really enjoyed my stay. So if you ask me, it's worth it and save enough. If you rent a car, 2,5 weeks will do to see everything. Less could be enough. But if you decide to travel with public transportation you will probably will need about 3 weeks minimum.