I usually try to fish (lake fishing) the same fly for as long as it lasts, as long as the fly is drawing strikes. I just assume that it should work if I get it in front of a trout. (Usually one of my Halloween buggers, a Sixpack, or a Dragon nymph, if I'm fishing sub-surface). This has only failed me a couple of times, when I just could not get a hit. Then I was forced to switch up. Still didn't get hits on those days. I blamed it on the atmospheric pressure, or something.
I will break a fly off and re-tie it if I see that my tippet has a nick or is getting frayed. I'll periodically inspect and sharpen the hook point, too. I fished the same "Failure Lake Special" about 8 or 10 trips in a row one year. Fooled over a hundred stocker 'bows and several native cutts before it got chewed down to a non-descript skinny scraggly nymph, which might have caused me to lose confidence in it and finally switch up, except that the lakes were warming up and it was time to switch over to searun cutthroat fishing mode in the tidal creeks here. So my local lake fishing time ran out before I was forced into action. I think I must have used only that one fly for all my lake fishing that Spring.
I have used one streamer for an entire Summer and Fall when trolling for searun cutts paddling in the estuaries and tidal flux here, too. It just worked too well to want to change to anything else. It finally got chewed down the following year, but continued to draw strikes, and eventually the hook rusted and broke at the bend and I was forced to retire it. (Of course, I also had a couple of other rigged rods along...one with a floating line for dries (possibly with a # 12 Montana Bucktail already tied on), and another with a clear-intermediate sinktip and a Reversed Spider.
The joys and advantages of being a simpleton are not to be under-rated!
I will try to match the hatch if I see surface activity, though.