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>How slow is too slow?

That depends entirely on your financial structure, your product, and your own lifespan. If you never on-board investors and never depend on a loss-leader strategy for capturing market, you can small-business yourself from you mid-twenties to retirement and beyond. A lot of trade professions do exactly this sort of slow expansion; my neighbors is self-employed plumber and he's been routing tubs for the past two decades.