If you're administering sub-q fluids daily or every other day, they will last several weeks. I'll use a bag up to a month and the same venoset for 2 bags. Changing out the needle after each use will prevent contamination. There's no need to refrigerate fluids - just keep them out of the heat and sun. Always warm them in the sink or a bowl of hot water (keep the needle out of the water). The best fluids for maintenance is Lactated Ringers. NaCl (sodium chloride) has it's place but it's not good for maintenance and can cause hypertension.
I do my cats on a towel on the kitchen counter and hang the bag from the top of the cabinet with a big S hook, actually a plant hanger from a garden center. Some cats like a little snack while getting their fluids. I keep my hand on the cat, petting around the head/ neck, ready to restrain if necessary.
Don't insert the needle in the scruff where there's muscle and can be painful, causing the cat to resist or fight the whole process. I go further down, in the saddle area, alternating sides and avoiding the spine. It's better to move around to prevent the formation of scar tissue. Good info here, with pictures:
http://www.weirdstuffwemake.com/weird/stuff/pets/cats/sophia/catjuice.html
Get good needles. Terumo needles are smaller, sharper and the thin walls make the fluids flow faster so it's more comfortable for the cat. The 1-inch length makes it less likely you'll go right through the skin and out the other side. You can get needles and infusion lines from Thriving Pets without a prescription. Fluids do require a script. I get fluids from my vet at their cost and you'll probably save some money if your vet will do that.
https://www.thrivingpets.com/terumo-needles-ultra-thin-wall-20-gauge-1-inch-box-of-103.html
https://www.thrivingpets.com/iv-admin-set-bbraun-v1402.html
https://www.thrivingpets.com/lactated-ringers-inj-usp-1-liter-bags.html
200 ml/ day seems like a lot to me and it's certainly too much to administer at one time. Too much can cause overhydration and pleural effusion which is life threatening. I'd start with 100 ml/ day. Fluids will flush out the toxins and help him feel better. Weekly B-12 injections can help improve appetite. Some cats eat better if their food is 'seasoned' with Forti-Flora.