The real answer is "it depends". It depends on your driving habits, the brand of shocks installed, the condition of your springs and I'm sure over a hundred other variables. In any case, if your car is starting to handle as differently as you say, then it is very likely your shocks are worn out. If those are original springs, I'd consider repalcing those too. Also, if you do go through shocks as fast as you describe, with the "dirt-roading", maybe get some lifetime shocks you could replace free the next time they go out!