Rulers-- I've never replaced any.
mat--after 20 years I did
rotary cutters--when the screw keeps jamming and I can't unscrew it myself and oil doesn't work
blades-- when the skipping gets on my nerves
pins-- I'm using ones that are 5 years old
needles -- after every big project I clean my machine and change the needle and add a drop of oil in the race
chairs-- when back pain is occurring after every sitting
machine-- only to upgrade or for fun (in both cases, only if I have saved the cash --won't go into debt)
scissors-- replace when lost or stolen by sewing gremlins
seam rippers -- when I'm doing more of the work that they are (i.e. When they're dull and I'm pulling more than they are cutting