Last year we put in a 22kw whole house generator, plumbed to our existing self-owned 500 gallon propane tank. Since that time I have been ruminating over run times vs. fill level & eventually decided we really needed to expand our fuel reserve.
The issue was that we normally wait until about 50% full to call for a refill, i.e. 100+ gallons on a fill to avoid a potential additional fee. At the 1/2 full point, I was estimating about 4 days - maybe less if we are burning a lot of gas in the furnace ( instead of running the 5 ton heat pump). Our most likely outages occur during severe ice storms - which also make LP refills potentially harder to come by due to road conditions so that scenario was reducing my comfort level with the existing tank.
Southern States was happy to order us a new 500 gallon tank and handle the install to parallel the existing one (other suppliers were not interested - they wanted to supply a "free" tank). Cost for the tank itself was about $1590, delivery & install about $300 (may be less, haven't seen final bill). They quoted $1.699 /gallon for the initial fill, which is a decent price around here at the moment for a residential LP delivery, $0.10/gallon cheaper than our other local supplier.
It took a week or so for the tank to come in. I prepped pads & gravel for the new tank while waiting, when the installers came out they tweaked them a bit & set the tank, filled it, then left for another job. I cut a slot on the old tank cap (mirroring the one on the other side) to accommodate a line coming from the new tank.
After a couple of days, the mechanic came by & set up a "T" connection for both tanks to supply the existing regulator.
The 1/2" line runs under ground for a space between the tanks. This was to reduce the potential for damage from things like falling limbs or running deer.
With the new tank, I can figure on better than a week, even with the tanks down to the 50% mark (longer if applying conservation measures).