For me, 30 minutes would be kinda short. But what's important is that everyone, consciously or not, prioritizes guitar and guitar practice against everything else during their day. Those that really want to practice, 'get better', etc., will find the time to match practice to their goals. Some find 30 minutes daily is enough... It's not for me; there are too many things I want to learn, too many songs I have to revisit, too many lessons I want to take. Besides, my 'natural talent' doesn't register on the scale...my guitar skills come slowly. :(

Efficient practice is very important. IMO, the shorter the practice time the more important that efficiency is. Make every minute of that 30 count. Already know, i.e., have on paper, what you're gonna practice and for how long. I do...it's like a mini-lesson plan...I guess I picked it up from living with a teacher for so long... ;) I count only the time my fingers are on strings...not set up, not dwelling on which lesson to bring up next, not tuning, not any of the time-sucking diversions common to guitar practice and guitar players.

Thirty minutes a day is a great start. Keep faithful to that and you'll improve. Practice more than that and you'll improve faster.