SE tires

We have a 2004 SE Offroad and I was wondering at what mileage did you guys replace your OEM tires. I swear we only have 18,000 miles and they are getting ready to be replaced. Isnt that a bit early? Did you guys replace witht he same tire or go for something else. She doesn go off road so even a nice quiet tire that does good in snow would be fine. Any suggestions? I know they are not cheap to replace. I have almost thought about buying a set of chrome 18 off off of ebay with the tires for summer and then go bcak to the OEM for winter. Does anyone else do that? Thanks