If you are really putting it through its paces, wearing it every day, taking it into the ocean and the pool and everywhere you go, probably 3 years, simply to make sure that the gaskets and seals remain fresh and functional (a service on the simple ones can be as cheap as $60, or maybe even less. Or you could do it yourself, they are not complicated movements).
Most people, with casual wear, could go 5 years or more easily. Many people buy vintage seikos that have not seen service in well over 10 years and still keep acceptable time and wear them anyway (just dont submerge them at that point as the seals are probably not water tight).
It is more fun to bond with a watch than to cycle through them. I have slimmed down my collection to pieces I want to keep for a long time, or hold to sell/trade for something truly special. These are the pieces I would be willing to take apart and service myself. You will shell out a bit for the initial tools and cleaner/lubricants (probably $150 or so), but you can easily service a movement with the wealth of service manuals and step by step picture tutorials out there. Its not rocket science, this technology is well over 60 years old, and you will open up a wealth of vintage restoring and modifying options up to yourself. Many people fund their watch hobby through the simple restorations and repairs they flip for a small profit. It is these people who could service your Seiko for cheap.
If you are just now getting into automatic and mechanical watches, congratulations! Its a wonderful and enjoyable world, and truly satisfying to own a well engineered machine that isnt tied to the world of electricity and electronics. It feels good to know I have at least one thing that I know will not crash, freeze, or run out of juice on me. I can set it down and pick it up five years later and it will still work just fine (you should try to run a watch at least once every couple months so the lubricants dont gum up).
My recommendation to you would be to explore the WatchUSeek forums and see the multitude of options you have in affordable watches, especially when you factor in customizing. Take the time to identify what you really like and want and save yourself the hassle of impulse buy-and-flip as much as you can (even though you will do it and go through it; we all do)