How do I light (and clear) a torch?

Solution 1:

From your Inventory page, you can add a torch to one of your Pockets. In game, press TAB to open your quick select, and select the torch. Press the equipment key to activate it.

I'm not sure how long they technically last, but in practice they seem to be unlimited.

Solution 2:

You have to assign it to a usable slot. Just go to your inventory and select the torch and drag it over to one of your usable slots (that's how you do it on PC).

Then you can just hit your usable button or go to your "quick access" area and select it. Geralt will automatically bring it out pre-lit.

They seem to last forever or until you put it away. But I'm not sure on the length.