I am an unusual case: I am LADA (Latent Autoimmune Diabetes in Adults), sometime called Type 1.5. I am autoimmune (and therefore insulin dependent) but I didn't become symptomatic until my early 40s. Also, my weight hovers around 160 lbs, so my comments do not apply to type 2 diabetes patients who are significantly overweight.
I use Lantus as my long acting insulin, and Humalog as my fast acting insulin. I receive a shipment of three 10ml vials of each every 3 months (so I use 1 a month). I use 30 units of Lantus a day, and a variable number of units of Humalog, averaging 25 units a day. Thus, 1 vial of Lantus lasts me slightly over 30 days, which is how long you should use a vial of insulin before replacing it.
Figuring out how many units of insulin to take can be difficult. This year I got a Freestyle Libre 2 glucose monitor which has alarms which will alert me if my glucose goes above or below preset limits. Fortunately, I am sensitive to hypoglycemia; sensitive enough that I will wake up if I go low while I am sleeping, but I can still get as low as a glucose reading of 40 point before I feel the effects, which upsets me and freaks out my wife. The alarm is set for 80 points, so I can take some carbs before I get seriously hypoglycemic.
I adjust my Humalog injections based on my glucose readings and my carb intake using a formula I have adjusted over long experience. I may take as little as 1 unit (if I just need to bring my glucose reading down) or as much as 12 units with a meal, but most of the time I take about 8 units with a meal.