I was never more frustrated with homebrewing than when I first started asking about starters. The frustrating thing is that there are so many different ways of doing starters, so you will get lots of opinions, not all of them consistent, and many of them contradicting each other.
Some people will take 5-7 days to let a starter go, some will get one going, then pitch it the next day when the starter is active. Both camps have some level of success with their methods.
Here’s my usual workflow with the yeast/starter question. I also use FastPitch because I’m lazy and it works well for me.
IF- OG is planned to be under 1.055, just make sure the yeast is relatively fresh, don’t bother with starter.
IF- OG is planned to be 1.060-1.070, either pitch multiple packs, or build starter:
1 can fast pitch, mixed with 1 can water
1 packet of yeast
Hand swirl, and pitch next day when its active OR
If there’s time, let it ferment out, chill in fridge, decant, and pitch the thick slurry (takes 4-5 days to do this)
IF- OG is planned to be 1.070 to 1.085 or so, do above, but with 2 cans of fast pitch
IF- OG is higher, do stepped up starters- make a starter for four or five days, chill, decant, then build up again. My last beers (1.100 and 1.080) I stepped up twice, then the night before brew day, threw in more starter wort and pitched while the starter was actively going. I got quick starts and pretty healthy fermentations.
That’s just my method, and if you throw it into a calculator, I probably underpitch by a fair amount. But it works for me. Lots of people probably do it differently. The way I look at it, any liquid yeast thrown into some sort of starter, either for a day or a week, is probably improving your pitch rate some. You’ll have to try a few different methods and find what works for you.