Mine takes at least 20 mins to fully warm up and cycle coolant at idle. I know that it can loose its cooling ability but im not sure about heating. you could go with a 160 deg stat witch would make it reach it operating temp sooner but thats only because the temperature is lowered.
I think these cars naturally take a while to heat up. Mine does.
If you do replace the T-stat, use the OEM specified replacement. Our cars are not designed for colder T-stats, and your car will actually run worse. The PCM sends inputs to everything based on engine temp. I would only use a slightly cooler T-stat (10 degrees F) if you are running a power adder. 160 is way too cold.
If you have a colder tstat than stock then it will take longer to heat up like the others said. If you have the stock tstat, it could be possible that its not opening. If you want to get crafty, get an oild stovepot and boil water. Drop the tstat in there and see if it pops open. If it doesnt, you need a new tstat. Or you can just spend $6 and get a new tstat and skip the boiling water part
i have a 180* and as far as the way it warms up hasnt changed it still takes a while but i did reprogram the fan settings when i did the swap so all it does is just a bit colder than it would wit the stock 192-195* T-stat