I buy maybe one bag of compost a year. But I am an experienced composter.
For seed compost I either make leaf mould [make in autumn year 1, it is usually ready by spring in year 3 after sieving in autumn year 2] or buy coir blocks. If I have both I mix them up.
For potting compost I sieve any compost I made the year before, from all the kitchen and garden waste. I usually sieve in the autumn and stash the sieved stuff to mature over the winter ready for potting on in the spring.
I have 3 tumblers which I fill with kitchen waste and when it is nearly done, I run it through the wormery to break it down further and add that [sieved] to the potting compost bin to mature after a few weeks. Then put the next tumbler in [1/3 in each tray].
If you want compost to use in the garden quickly, you can do the Berkeley method of mixing about a tonne of garden waste, kitchen waste, shredded paper, leaves - anything you can find and layer it up with water making all the dry stuff damp, then cover and leave for 3 days, and then turn it onto a pile putting all the outside stuff into the inside and the inside stuff onto the outside and then, every 2 days, keep doing that and you will have about 0.6 of a tonne of compost to use in the garden after 4-6 weeks depending on weather. So if you do that in Feb it will be good to use in April. Again you can leave it a few weeks to mature. I did this in January/February and had enough to fill my polytunnel beds in April when I put my polytunnel up.
I also do cold composting in daleks and empty them out in the autumn, I currently have about a ton of compost maturing from 2 of those ready to be sieved onto the polytunnel beds to top them up and will use the rest where my outdoor cucumbers and courgettes will go. Any left over will go on general veg beds.