my uncle has a quizno's, great clips, and postal annex store. the quznos shop cost the most, totalling over $500,000 to initialize (not including wages etc. to continue). its a lot of money, and he does hire managers for each of his stores, but he still has to do the most work. keep in mind not only do you need to equipment/organization/license from the franchise, you need rent/employees etc. as well.
not easy. not easy at all. and requires enourmous capital. not to mention a LOT of risk.
i dont think it would be quite feasible without going full-time but it may be possible in the long run once everything is well settled. but for starting out, its definately a lot of work. my uncle quit his high-tech job when he first started out. even now, it still requires a lot of devotion.