What are the safety demonstration time-constraints? On airliners, the safety demonstration includes many items (seat belt, life vest, evacuation, etc.) and each of those items may raise questions from the passengers (when to inflating the life vest, how to unlocking the seat belt, ...) and thus need more time to explain why those items are detailed. 

Given the following elements:


the safety demonstration should begin while leaving the gate.
an average passenger may not pay attention for more than few minutes
the briefing should be finished before take-off (I hardly see flight attendants moving and manipulating life vest, seat belts and other objects in the middle of the aisle while taking off)
many items may need a better (and longer) explanation


I have several questions:


Are there any regulations about the timing of the safety demonstrations?
Is it up to the airline (automated videos are quite easy to time)? 
What if the safety demonstration are not finished when the aircraft is ready to take off (on the runway with all clearances)?
I haven't heard of any regulations about the length of the safety demo, just the content, which is pretty much as you described. However it's in the airline's interest to keep it as short as possible. The aircraft cannot take-off before the chief of cabin notifies the cockpit that the cabin is ready for take-off. The cabin is ready after all passengers are on their seats, safety demo completed, galleys secured and hand luggage stowed away.