I'd recommending reading over info on Stumptuous if you haven't already:
http://www.stumptuous.com/category/training
http://www.stumptuous.com/category/starting
and specifically the workout ideas area:
http://www.stumptuous.com/category/t.../workout_ideas
The New Rules of Lifting for Women is also a really good book with a 6 month progressive weight lifting program.
Personally, 6-8 reps is pretty low, I'm more of an 8-12 rep girl myself with 12 reaching failure. I'm also a fan of increasing the weight with your sets.
I'm also not a fan of exercises for the smaller muscles. I'd totally drop the shoulder shrug and upright row (which is a bad exercise - prone to injury). I've never done shoulder shrugs in my life and unless you are a body building competitor, I'm not sure I see a use for them.
I also don't see any lat exercises?
My ideal program would include:
Pushups
Pullups
Squats
Lunges
Deadlifts
And then maybe some supplementary exercises. Pullups are difficult to do at home unless you have a pullup bar and the strength to do them. You can simulate pullups pretty well though with resistance bands and a place to attach them above you.
Pushups can be done on the knees but I like to do them on an incline like a weight bench or stairs.
I highly recommend planks, you can do front planks and side planks.
Ok and I didn't answer any of your questions!
1) How often do you need to switch - if you are doing the same exercises each session, then I believe 6-8 weeks is the mark where you should change things up.
2) As I said above, I'd drop the upright rows and shrugs, I'd add in some lat exercises. Bent over rows and pullups with bands are a good option. I'd also add in pushups or a variation. Also, are you lifting to failure? Are your arms/legs trembling on your last rep and feel like you can't do any more?
3) It depends. If the workout is strenuous then I'd think you'd see something within a couple weeks. More like you feel like your muscles are becoming firmer. I notice it doesn't take long for me to feel something. It also depends on your eating plan as well.
4) 3 hours sounds like a good amount of time but you may want to look at just making it more efficient. Concentrating on bigger muscle groups, lifting to failure, etc.