I drink about 5-6 beers per evening - that's 60-72 oz per day. At that rate, a 5 gal keg will last me about 9 days (on the low end, maybe 11-12 days if I stretch it). So to be "in stock" I need to have about 3 kegs ready every month. Not on the same day, mind you, but close to each other. Assuming that I keg condition for about 3 weeks before serving, and P+S is another 3 weeks, then I need to brew 6 weeks in advance. Assuming I'm still doing 5 gal batches, I will either need to brew double batches every other weekend, or else choose to brew 8, 7, and 6 weeks in advance for the upcoming month. Either way, though, at 15 gallons per month, that is 180 gallons per year.
To make it more difficult, I plan to serve my own beer at our wedding in June. That'll need at least 6 kegs or 30 gallons just for 1 day. Plus, I need to brew about 15 gal of homemade root beer for the same event.
I've been "running out" since the day I started. I hope to fix that problem next week with some super-mega brewing marathons, and further fix it by kegging everything I possibly can. I'll need a few more primaries though.