Can anyone give me a time recommendation on how long I should allow a yeast starter sit on the stir plate for before pitching? I usually make it the night before (about 18 hours prior to pitching) but am wondering if it should go for 24 hours + ideally?
I doubt you'll notice a huge difference. In principle there might be a slightly higher cell count after 24 hours, since the aditional 6 hours gives time for an additional budding, but at that point the limiting factor isn't time, but how much material is in the wort for producing new cells.
This is assuming you pitched 50-100 bn cells of viable yeast (e.g. a whitelabs vial).
If the yeast are not so viable, e.g. you pitched only 5 bn viable yeast cells, then you may need more than 24 hours - since there was only time to have budded just 3-4 times, giving ca. 40-80 bn cells. In this case 36 or 48 hours would be better.
I was just reading about starters, and for the life of me cannot remember where I read this, but if memory serves correctly, I read that the poster liked to pitch his starter between 12 and 24 hours, before the activity subsided. (Paraphrasing) The poster was willing to give up a bit of the yeast count for pitching while the yeast was still active. Otherwise, if you wait until it the starter subsides, the yeast has to be roused again. I'll try and see if I can recall where I read that...
Without careful monitoring of your true viable cell count at the time of pitching, its impossible to know exactly when the cell growth will max out.
That said there isn't likely any real difference between 18 and 24hrs in most cases.
For Ales I routinely go 18-36 hours and I pitch the whole thing. (I use a stir plate too BTW)
For Lagers I usually go 24-48 hours, then cold crash for 24 hours, decant 80-90% of the spent wort, pitch the slurry.