I knew about when I'd retire so decided to grab all the toys I thought I'd need beforehand while I could still afford them as I knew my pension would only be about half of what I was used to bringing in with all my over-time. Also collected a lot of fabric back then. Didn't realized just how much fabric I'd collected until I had to start packing it away as I was also going to sell the house and move back to the midwest.
Once moved, unpacked, etc. I continued to collect fabric, usually from Connecting Threads as their prices were right up my alley as all my quilts are gits to folks. When I saw a sale on backings on other fabric sites I'd grab them if they had what I thought I might need in the future for the quilts on my list. Also my quilting thread came from Connecting Threads, their cotton which is very linty as well as their polyesters. I kept a log of which colors I used the most and stock up on them up to 10 spools at a time.
Since Nov 2018 I have bought fabric until just last week when I needed some for a backing for a doggie quilt. I've only ordered embroidery threads which I can also use on my quilt machine so a plus there. I have ordered a bolt of batting but only when it's on sale. Just recently though I did order 4 king size battings while on sale as I have most of my battings already cut for the quilts in line to be quilted plus some left on the bolt so figured 4 more battings should do the trick for what I have yet to start, finish or dream up.
Since my quilts are all gifts and I don't skimp on quality if I can help it as I feel making a quilt takes time, emotions, energy at times so I want the best of me in them. I don't like to waste fabric either so will use cutoffs where I can on the quilts whether on the borders or the backings if I'm having to piece them together. As for the doggie quilt, the fabrics were leftovers from a larger queen quilt I made for a gal who suddenly passed on before I could give it to her. Tried to make her a lap quilt while she was in the nursing home but she went within a week's time. Found someone that liked the quilt and he has a big shepherd dog so thought why not make the lap quilt for her.
Anyway, I'm not rich by any means but I do okay. Not spending on my hobby these past couple years has saved me a ton of money though as I remember my expenses were tight back then.
My suggestions is buy when on sale, have a list of projects you'd like to make and keep a list of what you need whether fabrics, battings, backings and/or threads so when the sale comes around you know what you should be looking for.