Rulers - Replace if they get badly nicked or chipped, otherwise I buy ones in sizes I think I need when I come across a good sale.
Cutting Mats - right now I have 3, one is about 20 years old but still functional, one is the main one I use its about 6 years old and is starting to get score lines that wont self heal, will be replaced soon, the last one is brand new received as a gift, the same size as my 20 year old one, so if I ever retire the old one the new one will replace it.
Rotary cutters - I have 3, they all work fine so I dont feel the need to replace them.
Rotary blades - consumable product, replace often when the blade shows signs of dullness or nicks
Pins - constantly throw away the bent ones, when my supply starts looking low I buy new ones to add to it
Needles - consumable product. I change needles with every project and sometimes mid project. I buy them in bulk lots of 100 or more on ebay.
Chairs - never had to replace my sewing chair, it works fine.
Sewing Machines - have my machine serviced once a year it doesnt need replacing. That being said, there is a new machine that I really want, so when I get that then I will have 2 machines to alternate using.
Scissors - have the good ones sharpened as needed. The ones I consider to be semi-disposable (fiskars mostly) get retired when starting to dull and moved into the office to be used as paper scissors.
Seam Rippers - I buy 2 or 3 of my favorite style at a time. Have to argue with my husband when its time to buy more as he cannot see how a seam ripper could possibly get dull!