We called this practicing reverence and talked about it at FHE and during family scripture study. I'd say 5-15 minutes is good...start with 5 and work up to longer. With our kids, who are older, we talk specifically about what we are doing and why. With a toddler, I would recommend practicing with him on your lap while Dad reads scriptures and vice-versa, or spend time in a picture scripture book with him each day that you can then bring to church. I personally think it's better to do this when you are doing some spiritual activity. Maybe doing some modeling of church structure like you talk about would be helpful, but probably when he's a little older and understands what you are pretending to do.

When they are as young as your son, it takes a while for them to 'get it' but the more practice they have sitting still, especially during spiritualish activities, the better, IMO.

As they get older, I would also try to help them understand that 'reverence is more than just quietly sitting' (p. 31 of the Primary Songbook). We talk often of the importance of feeling the Holy Ghost and behaving so others can. We have to revisit this regularly, as they want to interrupt or distract or whatever during scriptures. (This is our best practice time, by the way, because it happens every day. But FHE is good too.)

Don't know if that is specific enough, but there are some thoughts. :)

I think we can slowly teach our children how to behave appropriately in this type of setting in many ways. We can help them understand to sit quietly at a movie, or at a restraunt, or when company is over, etc. I think the parent who teaches a child to be respectful in general will see results eventually in other areas of life. They will need this for school as well.

We do 3 things, in this regard.
1.> We have daily scripture study, which works just as well for "practicing church"
2.> If the child behaves poorly in the chapel, we take them to the hall, then force them to sit in our lap and be quiet in the hall, and not get up, walk around and play. If they can't do that, we go sit in the car and do it. This is hard, but it works.
3.> I try really hard to give my wife some respite in this regard. The children sort of gravitate to her, but I try to focus their attention on my, so she can focus. To be frank, having an hour with my four year old cuddled next to me and my 5 month old in my lap is exactly what I want to "get" out of sacrament meeting.

I found that you can never quite replicate the experience of sacrament meeting at home, even through practice. However, I did find that if we had 'quiet time' for a while during the day it really helped with this idea. In our house quiet time usually involved me putting on some music, maybe classical, or something else soft, and sitting down with them in a setting where they could sit on a couch or chair and giving them some books to read while I read my own book. This was different than when we would have a story time because they got the idea that 'quiet time' involved trying not to talk, respecting mommy's reading or thinking time, while looking a picture book themselves.

I started out slow and increased the time a little over time - so that in the beginning, just for them to get the idea, it might only be 5 minutes, and later it would increase up to as much time as I thought they could handle. One of my toddlers was a lot more rambunctious than the other two and he could not last nearly as long. When he couldn't sit on the couch anymore I would give him some really quiet toys to play with on the floor and tell him it was still quiet time so we had to be quiet. The other thing I added was putting on a radio program like NPR. Of course that isn't spiritual like sacrament meeting, but it is listening to a person talk - and it replicates the experience to some extent.

I found this helped a lot. But of course, depending on the disposition of your child on any given day, things can still go badly at church. However, I always found the best way to handle this was to take the child out when the got loud, go into an empty classroom with them (preferably one where you can still hear the service) and have them sit on a chair. They get soooo bored reallly quickly. Then I would say "we can sit here and listen quietly or we can go back in to the meeting where your toys are (cherrios are) and listen in there and in there at least you can see your family and friends. But we still have to be quiet right now, it's like quiet time." This works really, really well. It only takes once or twice for most kids before they decide it's way better to just accept staying in the meeting.

I had one really tough toddler, and two sort easy ones, but I never found it was impossible to sit through the meeting with a modicum of decorum. Now whether or not I got something out of the meeting until all my kids were past the toddler stage, is another matter, but I think it taught them something important about being quiet and respectful in church.

It's my personal opinion that parents make the biggest mistakes when they 1. punish their kids for being noisy or 2. let them roam wild in the halls or 3. let them go into a room with other children to play. All of those just create all kinds of new problems.

I'm thinking back to the days when I walked up and down the hall of my apartment carrying a fussing baby while my toddler clung to my legs. I felt like I practiced church all the time ; )

As I talked about this with my husband, we came to the conclusion that we hadn't done anything that could be classified as practicing church for our children thus far. So this is a comment to say that I have no advice to give in this area.