I have ground Widow for 2 years straight in practice room and finally moved onto custom bots. McCree was faster, I got half decent and started maintaining around 11% crit within about a year of practice. I personally refuse to seriously play widow in comp until my aim meets my standard though, since your aim ability in a way determines what style of widow you can reliably run.

I enjoy grinding away in practice or custom games and trying to slowly figure out what I need to work on. It's not for everyone. I dedicate at least 15 minutes to warming up before a comp game, and if I'm clinic'ing myself, I might go at it for 30 to 60 minutes. Just clicking heads, tracking, flicking, hard flicking, with or without movement, flipping back and forth between bots and custom.

It doesn't happen overnight and needs to be something you enjoy honing though, otherwise you'll just fall off the horse too much to see results.

I've been recording incremental clips if you want to see what aim progression looks like.