I admire and respect your dedication to continued improvement. One thing I have learned after many years of playing guitar is that no matter how good you get, there is always ample room for improvement. That's a good thing because it keeps the guitar fresh and interesting. I can continue to improve until I am no longer on the north side of the grass.
How quickly you make gains depends on how focused and intentional you are at improving your skills. From your question, it appears that you are deliberately seeking to avoid the trap many guitarist fall into where they learn to play a few songs they like and never try to challenge themselves to get outside the comfort zone they create for themselves - thus they never get any better.
For me personally, I find it more inspiring to develop new skills by learning to play songs that I will enjoy playing. I am always working on several that are more challenging than the ones already in my repertoire. By gearing my "practice" ("homework") around a goal of learning to perform a particular song I like and want to play, there is a tangible reward continually visible on the horizon. Once I finally master the difficult parts that allow me to add that song to my repertoire, I find more songs to work on.
I find that once I master a difficult lick or run in a particular song - I discover many more songs that I can suddenly now play - that I could not before - because they had similar passages or riffs.
This works for me because I play guitar to perform and to accompany my singing as both a solo performer and part of a band. Your goals may differ but even lead guitar players have a repertoire and building your list of songs you can play (rhythm or lead) provides a tangible and measurable sense of accomplishment!
One thing I have recently started doing more of that you might also enjoy, is playing along to guitar backing tracks. Getting the band together to rehearse or practice is logistically challenging - so practicing with other musicians is not something that I can do on a daily basis. But practicing or playing to a backing track is the next best thing.
You can find free backing tracks on line and download them to a file folder on your computer. Or you can create your own with some simple recording software.
The sites I use to find backing tracks are Guitar Backing Tracks dot com for Mp3 tracks or Betty Lou's Guitar Music Site which has MIDI backing tracks to go along with the lead sheets for thousands of songs organized by genre and searchable on the site.
There are free programs for converting MIDI to MP3 if you prefer to convert the MIDI backing tracks to MP3 - but I just store them all in the same file folder and play them on my computer with external speakers. I can also connect my computer to my PA and play the tracks through there if I want to.
Playing with other musicians or to a backing track helps with your timing and makes practice a little more interesting. You still have to work on the repetitive runs to master a given lick. But you can use Audacity (Free open source software program), to slow down the playback speed of any sound file without changing the pitch. Gradually increase the playback speed until you can play it at an increased speed - then you have it mastered!
You can click the link to download audacity if you don't have it already.
I wish you the best of luck and great enjoyment as you continue your never ending journey of improving your guitar playing!