I think the issue has less to do with how many artists/songs in your "more like this" category and more to do with how many songs in your genre are available. I created two stations, one for J-pop and one for '80s music. The '80s station plays whenever I want, no problems. I've added a bunch of artists and songs to the "more like this" section, and still no problem. The J-pop station is the opposite. I can play it for about 90 minutes or so if I don't add a single artist or song to the "more like this" category. If I add anything to that category, it cuts off sooner. I tried totally deleting and re-creating the J-pop station, and I noticed iTunes played the exact same songs in almost the exact same order. I think that means iTunes doesn't have enough songs available in that category to keep the station going. The more I narrow the possible playlist, the fewer songs available to play.
That's just my theory anyway. And, I think it's why some stations play fine and others don't. It probably depends on the iTunes music library in that category. I also noticed that just because an artist is available for purchase in that category does not mean it's available to play in iTunes Radio.
