Delaying obedience

After giving an instruction, how long do you wait for a child to comply? Sometimes the children wait to comply while they are finishing up a project or they ask to finish the page or to use the bathroom or to get a drink. Sometimes we give an instruction and then there is silence. We can't necessarily tell if they heard us. I feel like we've gotten in the habit of repeating our instructions since the children often delay to do as we are asking. If we add time pressure, they often get frustrated, especially the one child, and they wonder why we are trying to go so fast. They do not understand that we do not want them to need to go fast, but just to keep moving steadily. They've always had trouble with accomplishing things, even when getting ready to go away to somewhere they badly want to go. They seem to have no concept of time or being late or why it matters, even though they can tell time on the clock, etc. The other child often pretends she didn't hear. Would you wait to give any instructions until the children look at you? What if they delay to look at you until they finish doing something else? How much time do you wait before going on to the next part of the instructional process? The children end up wasting so much of the adults' time, especially when they are supposed to do a series of smaller tasks that have some down time in between while we work with another sibling (such as while both are doing math or doing primitive reflex exercises, etc.). Then they have a hard time doing their work when it's their turn for instruction, since they've had a mini break. How would you handle delayed obedience? I thought of saying that they must respond within a certain number of seconds, but I didn't know if this was a good idea, since time pressure can be an added stress and does not seem to be understood by these girls but time is a part of life. The children are often not in an oppositional mood necessarily and would do as we ask, just not right away , but this habit wastes so much time. The twins are ten now, and they need to learn this life skill of continuing to listen and continuing to follow instructions. Thanks for your advice.