judging from how it progressed in my cat i don't think you could expect more than 6 months (could be much much less). we waited so long because my vet wanted to, and i finally had to fight them to get her on insulin. at that point she had given up on eating, couldn't walk, and was severely depressed. it was horrible -- i don't think you want to go thru the same thing.
pleading with you to go to the vet. most states require vets to write prescriptions for any necessary meds, so all you would need is probably a single vet visit to confirm the diagnosis via blood/urine tests and get the prescription. we could help you with the rest. even if the vet wants you to, you do not have to get a blood glucose curve at the vet's office when you start your cat on insulin. we can walk you thru doing the curve at home once you learn to home test her blood glucose. (i'd just tell the vet i intend to start her on the standard 1 unit of insulin every twelve hours and reduce by 0.25u if she drops too much on that amount)
over a year's worth of lantus is probably about $10 a month if you buy the cartridges or pens via canadadrugsonline.com but you need a prescription to get it ($129 including shipping).
check out janet & binky's food charts (the new and old canned food charts at this link:
http://binkyspage.tripod.com/canfood.html)for inexpensive canned cat food with under 10% carbohydrates (go for higher protein ones and avoid ones with gravy or starches unless your cat's blood glucose drops sharply). you do NOT need to buy any prescription cat food from the vet. even friskies and 9 lives have decent canned food for diabetic kitties.
learn to home test. you can get a newbie kit for the cost of shipping, get a full rebate in the sunday paper (Walgreens or other pharmacy ads) for a blood glucose meter (the kind diabetic people use) so you end up just paying the tax on it, or go to walmart and pick up a cheap relion meter and test strips. other supplies you'd need would be lancets to get blood from your cat's ear ($7 for a box), cotton balls ($1-2 a bag), and syringes ($17 per hundred via hocks.com).
it's not horrendously expensive. the most expensive thing would be getting a curve done at the vet, which you could avoid by testing at home.
honestly, you can do this. many of us have been in your position before. once you get a routine going it takes only about 5-10 minutes a day to treat your cat--well worth it for the love she gives you.