I have a 125% hydration starter that I got from some chef friends of mine, but I've been instructed that if I care for it properly I should be using no more than 90g of starter from the base culture at a time. Lots of recipes I've seen suggest using much more starter than this (multiple cups at a time). How should I go from my small amount of starter to a larger one? Is it correct just to use a preferment to increase the volume? If so, how long should it be?
Increasing the volume of your starter is easy. Just add flour and water and let it sit at room temperature for until it becomes ripe (you can tell because the mixture will be very bubbly). The time depends on how much flour/water you add, what temperature you are fermenting at, how active your starter is, and what percentage of water you are using. Wait a few hours at least if you are significantly increasing the volume.
If you don't want to store the larger volume for your recipes, just increase the fermentation time for the recipe. The only caveat is that you shouldn't try to increase the volume of your starter by more than a factor of three at any one time (and keep in mind you'll need to reserve some sponge for the next batch of bread). Thus if you have very little starter, you may have to triple it, wait for eight hours, then double it, wait, then use, for example.