Your speed in the boat is not only affected by the wind speed but also its direction. Running straight downwind is going to be slow, no matter how much sail you put up you will never reach windspeed. On a broad reach you can go faster than the true wind.
The answer to your question is relative to the amount of hurry you are in. I've found that if my speed goes below about 2-3knots I now turn on the engine and motorsail, but that is personal preference. When crossing an ocean the amount of fuel
is limited, so sailboats stuck in the doldrums with nary a breath of wind for days upon end sometimes don't have the fuel
reserves to turn on the engine but must sit out the still air.