I find a reasonable amount of time to practice is a 45-minute set broken up with 15 minutes of rest (or less structured playing) if you plan to go longer. (Our brains tend to lose focus and we don't make as much progress without breaks.)

That said, if you don't have time to do more than one 45-minute set, just aim for 45 minutes. And if you can't do 45 minutes, at least practice a few minutes each day.

For the first 3 years I played it wasn't unusual for me to practice 8-12 hours a day, because I was a teenager and had nothing else to do. I was either doing scales, finger exercises, or learning songs out of magazines.

As an adult, I try to practice a couple hours at night (2 x 45 minutes sets with rest / loose playing in between). My focus lately is entirely on sight-reading, so I'm logging all my practice time and progress.

Whether you will be able to write a song by yourself after working through Guitar Fundamentals is totally variable depending on the individual. Some people can write songs knowing even less. But songwriting is a totally different process than just playing guitar.

Also, being able to hear a song and play it, that's yet another process (ear training / playing by ear). Some people can do this very quickly and some require lots of work.

The courses on GuitarTricks are more focused on playing the instrument rather than songwriting and ear-training.

-Carl.