No one has really mentioned the type of fishing you do in regard to sharpening. When nymphing for example, where the fly is hitting rocks I tend to check every 4 to 5 casts, If I hook bottom, for sure a solid sharpening after that, or usually a change to a sharp fly. Fishing streamers will also dull a hook nicking bottom or wood. Fishing dries is a different matter, I don't check them as often as they are either in the air or on the water film. If my fly attracts a bush or grass, I usually do a check.
Met a young fellow on the water who was complaining lots of short hits with no hookups. Checked out his nymphs and they where the dullest I've ever seen, but wouldn't take a bit of advice to touch them up. Funny how people will spend thousands on equipment and not address the one bit that actually does the most good, the point of the hook. I believe dull hooks will harm a trout, instead of a solid quick penetration, you can end up ripping before a hook penetrates if at all. Ive seen these ripping gashes on trout and I have too much respect for our fish to do that.
I like my hooks sticky sharp, a great many flies you buy or hooks for tying are sharp but not to my liking. A couple of swipes is enough. At the end of the season during the winter is a great time to go through all your flies and touch them up for the season as a start.
You can use stones and files, I use a small Diamond Flat File. Move the file towards the point across the bottom and two sides to form a triangle toward the point. You're looking for just a sharp point not to file down the integrity of the point, if you get the sharpened area too long it may fold or curl. The time to practice this is at home not on the water.
Lots of good sharpeners out they but they are useless in your pocket.