I've been playing acoustic guitar since about September, and I got an electric guitar at Christmas. I feel like I should be playing better at this point, but I'm honestly not sure if this is the normal rate of progression for a guitar player at getting better or what. I'm able to play a fair amount of stuff, like the first solo in Master of Puppets (though not the rest of the song lol), Achilles Last Stand, and Stairway to Heaven, as well as a variety of other things (Hangar 18 up to the second solo, etc.) But I feel like I should be doing better at this point. All I know about music theory or anything like that is what I've learned from singing, which sometimes applies to guitar and sometimes doesn't. Does it seem like I'm moving along at a normal pace, and if not, what's a good way to get better?
seems like a fairly decent pace to me. Singing along does help some people. Just remember it takes time, a lot of it, so when you feel like you're stuck in a rut, work on a different aspect of music such as scales or theory or playing with other musicians. Basically, just keep at it. You sound like you're doing alright.
How much better would I need to get before considering joining/making a band?
Depends on what type of band. Do you wanna do simple power-chord punk'ish songs, or do you wanna do metal, or classic rock? It all depends on what style music. Just make sure you are confident in your own abilities, and can sit down and play that style music without even thinking about messing up.

As long as you're confident, and play a style that's within your ability, you'll be ok. You can start with a basic rock band and as your playing, as well as your band mates' playing evolves, you can get into different sounds and more complex arrangements.
Damn, dude! That's some fine progress. I've been playing for nearly a year, and I still can't pull-off Smells Like Teen Spirit's solo! Guess I'm better off as a rhythm guitarist.

Keep practising, and never give up.
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As long as it sounds good to you you shouldn't really care about what other people think about your progress. Find some people to play with even if they do not play metal. Be open to other music. You'll become a much better player by playing with others regardless of their style.

If you want a challenge pick up a copy of Shut Up and Play Your Guitar by Zappa and try to play along with it. Just be sure you ignore everything you have learned when trying it.

Good Luck!
"Communism doesn't work because people like to own stuff."
- Frank Zappa
Well depends, you could always find band mates that are still learning too, that way, you could all become good together, so you dont have your drummer playing YYZ and your still getting down the progression from a F to A minor...
"Guitarmen, wake up and pluck wire for sound, let 'em hear you play"
-Charlie Christian
"You have to give people something to dream on"
-Jimi Hendrix
"I try to make any guitar do what I want it to do"
as long as you can keep time im sure you can find a group to play with but thats the most important thing (I think) when playing with a band