how long do coils last for a motorcycle, buying one used?

Like someone said a crap shoot new or used. I have had coil failure on a brand new products, run for about a minute. I had one go on a new snowmobile at 25 miles. I have seen them on a 60 year old machine literally rusted to it and still working. If you do buy a used coil do a resistance test before you hand over the cash. If it checks out good, you have nothing to worry about.

Apparently about 30 years. Please make sure it's actually a bad coil before you spend money on a new or new used one.

If you can test the new (used) coil before you buy, do so. If not, it's always buyer beware.

I don't have much experience with Japanese coils, more with BMW stuff for those years and before. There were certain coils in the BMW line that basically lasted forever and had few, very few problems. Others would have a significant number of failures within 30 years (cases cracking). Still, a coil that lasts 30 years is pretty damned good.

Buy the new coil. You just have no idea what you're getting with a used part (especially an electrical part). It's not about the relative price of the part and the machine. You can't buy a better bike for $200, so you'll want to keep this one running...

The best answer is five minutes or from now on for a used coil. You can probably find a Honda dealer with a brand new one sitting on their shelf that they would sell for just over cost to get off yheir inventory sheet. Yes it would take making some phone calls but then you'll have a brand new coil for the price of a used one with condition unknown. You might even get lucky and find a Honda dealer that's willing to take on that search for you. You might be fine buying used but if you can get a new one it's always better with electrical parts.

Electrical parts can be a crap shoot, new or used. I had to replace a coil on a three year old Yamaha and my sons 1979 Suzuki still has the originals on it. Bite the bullet and buy a new one.

that depends on the bike.