Culligan ZeroWater filter lasting 5 months?

Purchase the 7 cup water filter pitcher on Feb 1st and the TDS meter still shows 0, If I put just straight tap water in it'll go to about 23. the taste of the water hasn't changed but I've seen post of people saying it should last for about a month? Just curious if anyone else has had a filter that's lasted this long or should I be concerned lol