"My laptop is of course 12v"
Are you sure? that's quite an unusual voltage for modern laptops, 18-19volts is more common.
Laptop power supplies can pack up, just like any piece of electrical equipment, but in my work I'm responsible for about 100 laptops and I've had to purchase replacement power supplies for 2 over a 4 year period and I'm sure both of them have been due to the cable going to the laptop getting broken where it enters the PSU and cables breaking is pretty much guaranteed to be due to "user abuse" (i.e. winding the cable around the PSU too tightly repeatedly).
So basically, take care of it and it's very unlikely to fail, if it does by some chance generally universal power supplies or pattern ones are far cheaper than genuine ones and do the job just as well, I've never had to pay over £25 to replace a power supply.