How often you need to replace your running shoes depends on how long you have been running in them, and how many miles you have put on them. Certain people find that their shoes wear down faster than others begging the question of how many one should have. This is typically because of the specific way you run and your individual foot strike, as well as factors like size and weight. Most experts agree that 300-500 miles of running is about when shoes need to be changed out. For some runners, that's only every couple of months and not that big of a deal. But if you're sticking to a high-mileage training plan or training for an endurance race like a half, full, or ultra-marathon, then you'll log 300-500 miles more quickly than you can afford to buy a new pair of $100+ shoes!
For this reason, it's a good idea to keep the "300 miles" rule in mind, but to know yourself. One way you can tell that it's time for a change is that you start to experience foot or ankle pain that typically isn't there. If it's pain from needing to change your shoes, it'll feel like a dull, aching pain because you have worn down the support and cushioning in your shoes' soles. Another way to check for "shoe doneness" is to test their flexibility. Most training shoes are cushioned, but also stiff and sturdy. If you can easily bend the toes of the shoes back, it is time to switch them out. Similarly, if your shoes are showing other signs of exterior wear and tear, such as holes, and stretched-out heels, you need to replace them.
If you've been running in the same pair of shoes for a year and a half, it is probably time for a new pair. Test their flexibility and then I advise going to try on a new pair. It might be that you have gotten used to the way your tired, old shoes feel and that once your feet are in something new with lots of support, you will know that your old shoes aren't actually giving you the stability you need.