That's almost a how longs is a piece of string question. Some can fail in a matter of years others still working fine and everything in between. As a general rule it's more mileage related than time.
As there's only two coils with 2 HT leads it would be 2 cylinders not firing, not just one, if you had a coil issue.
If you've had issues which prompted you to remove the carbs and clean he pilot circuit? Chances are it's still that, that's the issue. Does the cylinder fire and run/rev above 4k? If so it's not the HT circuit, it's the carb.