Budget big, live small, and you can go indefinitely. My budget is a worst case scenario budget, which is going to exceed most cruisers' DAILY needs... BUT allows for a cushion for eventualities such as engine/sail/catastrophic repair replacement.
How many times have you seen boats abandoned because their owners tried to live too small, and didn't account for big expenses in their budget? yeah, you won't NEED $5k every month, but one month every three or four years you WILL NEED $10-20k to replace a dead engine, new sails, new standing rigging, etc. If you haven't allowed for those costs, you're going home, likely without your boat.