Hi simpleman9
How long is a "piece" of string?
Over the month, how frequently is "regularly", for how long per session, and do you actively focus on the lesson objective when practising?
Unless you have a physical disability e.g. arthritis or injury?, there's no reason I can foresee you shouldn't eventually manage it [u]if you want to enough[/u]? However, it does sound like you might like to look to 'sharpen your learning tools', or practise a lot more until it's rote, possibly both.
Here's one tip. For a clean minimum fretting pressure of the G, B & E strings for D, ensure your fingernails are trimmed short.
As for how long it should take. As long as it takes. What's normal? I can only offer my own experience as a guide, and I wouldn't assess myself as other than making average or normal progress.
I had nailed the E-A-D change within a few days, to the point of changes being 120BPM tempo-ish riff/song fluid within the week. I play A using the alternative common anchor fingering mostly, advantaging the various A-D-A-E-A-D-A sequential changes. You can try it here. But, that said, I find the change equally easy using conventional open A fingering as taught on GT by Lisa. I learnt conventional A fingering originally 45 years ago, but hadn't played it, or at all, over the intervening years, so relearning Justin's preferred contemporary fingering came easy. You'll see how alternative fingering's anchor finger relates to conventional E & D fingering facilitating the change between them.
Regardless of the one you choose to use, it's still comes down to repetition forming the chords properly. Slowly at first until the forming becomes motor skill acquired and automated, then speed will build. It's easier on electric than acoustic because of the lower action, and generally lighter string gauge. Is your guitar strung with 9's or 10's? Is the action within spec?
[br]As incentive, a top riff you can play on electric using just A-D-E occasionally throwing in a G-A-G-A (search for the actual song and chords online) is The Troggs "Wild Thing".Crank up the crunch and let rip.
Utilising a riff or song which appeals as motivation to accomplish fluid chord changes I found an effective way to master getting them fluid pretty fast.
Similarly G-D-G changes. I 'fess I found fingering open G - any variant, easy peasy from the start. My fingers/hand just found it a natural feeling chord for me. Could play them shifting to and fro within a typical progression easily within a week. G-Em-C-D-G (or easier change than D, D7) for four bars followed by C-D-G-E, rinse and repeat.
[br]Some chords will present as easier to learn than others to each person.
Or G-Am-C-D for several bars followed by C-Am-C-G, C-Am-Am-D will play you "Love is all Around" (The Troggs '68/Wet Wet Wet '92). I think that took me about a three or four days to nail as the first actual song I learned when I returned to relearn last November.
The full open four finger fretted F is probably the most difficult open chord personally for me to change to and finger rapidly and accurately consistently (due to the required forefinger angle and pressure). Although I can do it faster, better and easier, even the contracted three finger verson dropping the fretting of the A string I still find uncomfortable and unnatural, so have to really focus and work on it all the time. I even find changing to a chord like B7 and barre Bm easier and comfortably faster.
[u]Keep at it[/u]. You can get there. Just review your application in how you are practising, and try to analyse what your particular impediment to progress with that chord/skill is. Having identified it, you can take the necessary corrective action to deal with it. Remember, obstacles are part of the fun of the journey. It wouldn't be any fun at all if it was without them or easy.
[br]Good luck.