Yeah it really varies. Some attys burn more liquid per same-length-drag than others, cartos may burn more per drag that the average atty, and everyone's drag is a different length, everyone has a different size mouth and lungs, etc. When i started I used about 1-1.5ml per day and now I vape more and use up to 2ml per day. Some people use 3-5ml per day.
You can tell how much you use if you are using juice from just one bottle - just look at how much the juice level goes down each day, how far it goes down in 5 days etc. how many days did you use the 10ml bottle to get it to 1/2 empty? Divide 5 (1/2 the contents of the bottle) by number of days and that's how much you used per day.
re 3 months worth of juice - I don't recommend buying a huge bottle of juice without trying it. Every vendor's juices are different, every flavor is different, some may not have flavors you like nor whatever TH you need etc. Re cheap juice - if you actually like Dekang 100% PG juice and it doesn't make your tongue sore etc (100% pg lets through TH well but makes my tongue sore after a while and that ruins the flavor) then two cheap places for Dekang are LiteCigUSA.net and I think Liberty-Flights.com. I heard Madvapes has cheap large bottles of their juices but some of their juices have very strong aromas you can smell right through the bottle (anything with cream or spices etc) and can actually pass that aroma on to nearby juices, stuff near where you stored it etc. Excellent prices though, both for small bottles and large - excellent. I think their 5ml bottles may be under $3? and I heard they have a great price on large bottles. I just bought a bunch more to try (and also stored their more smelly flavors and my smelly gourmet vapors ones w/ nutmeg etc all together in a butter-spread tub, away from my other juices)
And not as cheap but my favorite juice sellers are FreedomSmokeUSA.com (code freedomsmokeusa usually gets 10% off), Backwoods Brew, and for lovely sweet ones Velvet Vapors (I think they reopened this weekend).
And if you really need 3 months worth, be sure to buy spare attys and and spare batteries, and spare carts.