Not an easy question to answer...
In all the sailing I've done, the wind has never been EXACTLY the same. Something is ALWAYS different. And remember wind can blow more directions than are on a compass. It can below up and down. I've had the wind directly below down on me flapping the sails and tracking the water, -and I'm just not moving! With that said...
It's personal choice of what you can tolerate in light winds. If it is alternating between dead calm and the occational variable puffs, you WILL get where you want to go, but you must be patient. Just carry a paddle, if you think you could run out of time. As long as the sun is around to to create uneven heating, you will move in a dead calm,--but the side slip is annoying. If you are caught in a slow wind shift, the wind will eventually return. There have been times I just give up on a slow wind, then just as I an lowering the sails, the wind freshens up and makes me regret my decision to call it quits.
Yes its frustrating, but if you want reliability, get a smelly motor and listen to the noise. --Wait a minute, that why we LIKE sailing. (Ever pass a motorboat on a windy day? The stunned motorhead just stares at you in disbelief that you can better his oil-and-gas belching beast. I just LOVE that.)