That's why we say 75 hours...but remember with LIVING MICROORGANISMS, anything we suggest is just a "rule of thumb," or average, based on our experience, but that doesn't mean the yeasties don't have their own timeframe and agenda...they are the beer bosses, not us.
Remember, if you have sanitized and sealed up your fermenter even if it is over 72 hours your wort wont be ruined if it takes longer. You can buy these and yeast wort kits in some shops. It's basically a 2 or 3 soda bottle of wort that you open, pitch yeast in and snap an airlock on. And those sit for many months on store shelves. And although I am sure it is not the greatest beer in the world, the wort doesn't instantly go bad if they are not sold within 72 hours of being sealed up.
So don't automatically assume just because the yeast doesn't appear to take off for over 72 hours, that it is automatically going to spoil, if you haven't opened the fermenter.
And under most circumstances your yeast will take off...UNLESS you have had mail order liquid yeast sent to you in the heat of summer and didn't make a starter, OR you dropped the yeast into boiling wort.
It is not like the bad old days of pre/1978 brewing when yeast came in dried out cakes that may have travelled in a hot cargo ship over months from Europe. Most yeast today is lightyears healthier than it was back then....
And even if some of the cells are dead, the first thing the yeasties are going to do when presented with 5 gallons of food is have an orgy and reproduce.....that is what is usually happening in the so called lag time...the yeast are waking up, and beginning to raise an army to tackle all that food.
If we can grow a huge 1/2 or 1 gallon starter from the dregs in a bottle of beer, then the yeast you pitch will take off eventually....and if you leave your fermenter alone...don't even look at it, in fact if you trust it...it
will be fine.
They've been doing this fermentation stuff for over 4,000 years in some pretty hostile condition, they should be able to handle your nicely sanitized and sealed fermenter.