HOW MUCH CLUTCH?

This is a real "how long is a piece of string?" question. A clutch can last up to 100,000 miles (160,000 km) or more with gentle use an correct adjustments. But a ham-fisted rider can destroy a new one in a few minutes.
If your clutch is ok, don't replace it. The firste sign of wear can be if it becomes grabby - that can be reson enough to replace it. And if it starts to slip (you'll notice it first when you give the bike a big handfull of revs in a high gear), then it's definitely time for a new clutch.
As for "do it yourself" replacement, it's a medium-difficulty job. If you do it, consider replacing the springs as well as both sets of plates (friction and metal). Genuine ones are better than replicas, but cost more too - lots of people use quality pattern parts such as EBC with no problem. You'll need to drop the oil out, remove the side casing and pay attention to how everything comes apart so that you'll be able to put it back again. Everyone says that plates are all the same size but I'd suggest to keep the old ones in the order you remove them as there can be subtle differences. And you can always use them as refference is needed.

I will publish an article about how to change your clutch yourselves in a few days. I am still working on it. Peace!