One of the most frequent questions I get asked from fly tying students is, "how long does it take to get good at tying flies"? Without going too much into how "relative" a good fly is, the issue really comes down to getting good at fly tying or getting good at tying a particular fly. The thing about fly tying is that even though certain patterns are similar and maybe use the same materials, unless you are very comfortable with the materials it can take a long time (and many flies) to become good at tying that particular fly.
If you have a small number of patterns that you love to fish with and would like to get good at tying (good meaning quicker, better proportions, and better looking) then you will need to sit down and tie a lot of those flies to get good at them. In the process of tying just that one fly, say two or three hundred, you will be learning what you like and don't like about the materials, the thread you are using, the hooks, the order materials are tied in and how the fly is finished. You'll even be learning more about what bobbins you like better, what scissors get the job done for you, what hackle pliers work best for you and what head cement you like best. Now you have that fly done but what about tying it in a size 18 instead of a 16 or size 20? Crank out another one to two hundred in those sizes and you will be even better.
But let's just say you love tying all kinds of flies. Saltwater, bass, nymphs, dry flies, classic streamers, Pike, bluegill, wet flies and on and on. You certainly can get good at tying flies in all of these categories but even if you are tying one hundred of each pattern it will take a long time for you to build up the reservoir of experience with all the different tying techniques, materials and designed to be "good" at tying these flies.
The recurring theme here is tying lots of flies. I recently had a project to tie 100 Woolly Buggers for some middle school students. I sat down at the vise with all my materials laid out and started tying. The first dozen looked OK but not consistent. The next dozen better and the next even better. What I found was that by the time I reached 50 I was starting to put the thread in the exact same place on the hook, I was starting to put the same amount of thread wraps around materials, I was picking out consistent feathers and wrapping chenille the same number of wraps with my head being the same size. By the time I reached 100 I was even incorporating a number of shortcuts to streamline the tying process and my flies were even more consistent. Imagine if I had tied two hundred or three hundred? And the Woolly Bugger is one of the simplest of flies.
I read once where Dave Whiltlock had set a goal for himself to tie at least one fly a day to get better at tying. I have had students who have done this and for them it has helped. A. K. Best, in his book on production fly tying, stated that it may take him 100 dozen to really learn a pattern.
As I mentioned above, a good fly is a relative thing. Often what we consider good is way more perfect than the fish needs. But who ties flies for fish anyway?