I think it depends on the conditions etc of your mowing. I mow a lot of grass and the soil is dusty a lot of the time so i grease my deck every few hours. Grease is cheap and it takes only a couple of minutes. Replacing a spindle or wheel would cost a lot more.
I got in the habit of greasing my deck every 4th use which equals about 12 hrs of running time. I rather grease it then replace parts in a couple years.