I have around 3200 miles on a Dahon D7 I bought one year ago. I regularly oil it every 150 miles or so. Shifting still seems fine, so when should I replace it? If it ain't broke, should I still fix it?

measure it. If it has stretched I`tll start wearing out your cogs and you may need to replace the whole drivetrain.

+1 on the measurement

It's hard to go just by mileage. 3200 miles without rain, dust, sand, or other crud might be just fine. My commuter bike, which sees a lot of rain, dust, etc. seems to need replacing every 1000-1500 miles.

Speedo

How do I measure it, any special tool I need to buy? Are they accurate?

I think there is tools, but you should read about this first. Try Sheldons sites or Park Tools.

Each link of the chain should be 1/2", so 24 of them should be 12". Try it. If 12 links measure more than 12 1/2" or so, you might think about changing the chain.

A worn chain doesn't seat properly in the teeth of the cogs, which isn't good for the teeth. After 1500 miles or so the chain on my Downtube Mini was so worn that it could unseat itself from the rear cog. Tightening the chain, by moving the wheel back in the dropouts, didn't solve the problem. When it got so bad the chain would come off every couple of miles, I changed the chain; and that solved the problem.

Obviously, if you have this problem, you should change your chain. If you're not having a problem, it's less urgent.

On a chain, maintenance is the most important aspect. Check this out>https://sheldonbrown.com/chainclean.html

I am interested in what Makeinu would say about this, since he declared

For me I'd go with the mentioned method, that of measuring the chain. And yes, do clean and lubricate your chain regularly.

I chuck a chain sooner rather than later. The cogs last longer that way.

A chain wear measuring is quite cheap and is worthwhile if you do a lot of cycling.

I'm a great believer in Purple Extreme. It lasts and isn't dirty.