Nicotine base stored in a freezer, in glass bottles filled close to full, should readily store for several years.
Personally, if you don't have sensitivity to PG, I'd go with a PG nicotine base -- it's just simpler to work with, because it's thinner, easier to measure accurately.
As to strength, if space isn't too big a concern, I'd stick with lower strength than 100, probably 60 mg/ml. I go by price point, how many mg of nic per dollar, and 60 mg/ml seems to be a sweet spot.
How much depends on what you vape. Say you vape 4 ml/day, of 20 mg/ml
eliquid. That's 4 x 20 = 80 mg/day of nicotine (you only absorb a fraction of that). A 500ml bottle of 60 mg/ml is 30000 mg of nicotine, or 30000/80 = 375 day supply, a bit over a year even if you don't cut down your nic strength over time (many of us do, but not all). A full liter would get you twice as much, of course.
A 500 ml bottle of nic can be readily split into 4 "120 ml" glass bottles for long term storage. The bottles actually hold a bit more than their official size. You only take one bottle out at a time, your working nic base.
For me, I recently went for 500ml, plus some cobalt blue glass bottles, from RTSvapes. They are a good supplier, seem to be decent on stock, though there are other popular nicotine base vendors.
You don't actually have to work with the 60 mg/ml, if you prefer not to. You can cut it to a lower level after taking a bottle out of the freezer, letting it get to room temperature. Take a 240 ml clean bottle, pour a "120 ml" bottle of nicotine base into it (assuming it's really 124 ml, 1/4 of a 500 ml bottle), then add 62.5 ml of plain PG or VG. Shake this bottle and you have a 40 mg/ml bottle of working nicotine base.
Hopefully this helps. Personally I wouldn't bother with more than a couple year supply, which is well within reasonable freezer storage time, but that's just me.
