As RiverCityBrewer stated, you need to have a vessel large enough to reach a certain cell count. Think of it as squeezing as many people into an elevator or phone booth that you can, only so many will fit before space runs out. The same is true with yeast, they can only occupy a certain amount of space in your starter vessel before they run out of room to continue to grow and reproduce.
I have had success making starters for 10 gallon batches in a 1 gallon growler, but it can get messy:
Stepping your starter-up is one way to get around using more yeast packs or vials, but you definitely need to plan your starter a few days in advance. If I'm brewing on Saturday or Sunday, I'll usually get a starter going Tuesday night or Wednesday morning. It gives me the opportunity to let the first 2qts ferment to high kraeusen usually ~18-24 hours, then I pitch my 2qt step-up for another ~24-36 or until completely fermented out. Afterwards it is into the fridge for at least 24 hours, but I like to let it chill for 36-48 hours. Decant and pitch on brew day.
I have had lag time of as little as 3 hours.
MrMalty.com is definitely one of the best sources for all things starter related, not only the cool calculator, but Jamil has some great tips on how to make a starter properly.
I am going to be purchasing a 5L flask whenever I can find one on sale. Unless anyone out there has too many and would like to donate one!