The shave test is the thing to go by. When the razor no longer performs as you expect, then it's time to consider touching it up a bit. There really (IMHO) is no other reliable way to make that call.

It probably will often need to be refreshed more often for a beginner simply because there are things that people are less prone to do as they gain more experience that can lead to a razor needing to be refreshed or outright re-honed. Rolling the edge while stropping leaps to mind as a good example.

As for whether to get a barber's hone or some other finisher, either one will do the trick. One thing to watch out for with barber's hones though is that you will often see older vintage ones, so you have to be aware of a few potential problems to watch out for (is it chipped, has the original surface been lapped away, etc). The advantage is that they are often cheap, so if you shop carefully, it can be a good way to go.

I'd also suggest considering a high grit synthetic such as a Naniwa 12k. I don't have a Naniwa myself, but I know a lot of guys swear by them. Natural finishers can be great, but you have to be a bit careful with them too due to natural variations, particularly with the Chinese 12k/15k/PHIG.

Personally, I go to my trusty Zulu Grey or barber's hone (Swaty), whichever I'm in the mood for.