If you are using a stir plate it will take anywhere from 24 to 36 hours for the starter to be complete. The starter will go from looking golden or amber to a creamy white as the yeast grows. You will also notice a small krausen (depending on yeast strain) and CO2 bubble gasing off along the edge of the starter and in the vortex. For some yeast strains, I’ve had the starter completely gas off and be done in 24 hours. Others after 36 hours it will still have a krausen and gasing off and I let it spin a little longer. I then cold crash the yeast for at least 24 hours but like to cold crash longer if the yeast strain does not flocculate well. On brew day, take the starter out of the fridge, decant most of the liquid and let it warm up to pitching temperature. Swirl and add to the beer. Some yeast strains like to stick to the bottom so you can add some chilled wort to help swirl it out. I have a plate chiller so I’ll sometimes run some chilled wort into my decanted starter and spin that while I’m filing the fermenter.
So if I’m planning to brew on a Saturday or Sunday I usually make my starter Monday evening. I figure by Tuesday evening or Wednesday Morning I can cold crash the starter. If the starter is not finished and I cold crash Wednesday or Thursday morning, I still have a few days to cold crash before the weekend.