I've sold quite a few shirts, but mostly when the store first launched; probably readers who wanted a shirt already got one, and I haven't uploaded any new designs recently. As for the time investment... I think it's worth setting up the store but don't expect to make huge sales out of it, and I would definitely not expect the sales to be consistent over time. The good thing about the merch store is that you set it up once and then if you sell something great if not, oh well, all you spent was some time.
Maybe you can ask your readers which ideas they'd like to see in a shirt? In my case, the best selling shirt is the one with the most complex design (a space station in my comic, which people have mentioned they like), so I'd say definitely listen to your readers.