I am making a 2 step yeast starter using intermittent shaking and its been going for about 18 hours and I woke up to a good amount of sediment at the bottom and a lot of bubbles still coming to the surface. Should I wait for it to slow down or just cold crash and move on start the next step up.
When you swirl the flask (or whatever) and not a lot of new bubbles are coming out of solution, then you know it's done. When I do multi-step starters, I don't crash before they are done. You could, but that would defeat the purpose of whatever starter sizes and number of steps you planned, i.e. to target "X" amount of cells at the end.
This is normal, but go ahead and shake vigorously. You want to drive off the CO2, mix in some air, and keep the yeast in suspension . . . all jobs normally handled by a stir plate. Time is a starter's friend. You may have a nice krausen (or it might come and go at night) or you may not. Yeast enjoy trying to annoy the brewer. As long as there are signs of fermentation (i.e. CO2/foaming) the yeast are still reproducing. Don't be afraid to shake it a lot, every shake makes it more efficient. As reproduction slows down you will notice much less off-gassing. Then at some point there will be no foam produced from a shake . . . this is when you are done. I normally give it another couple of hours to be sure. To make a two-step starter without stir plate could easily take a week (including chilling/decanting).
I am going to sort of disagree here...for a single step non-stir plate starter, 24 hrs is usually plenty...IMHO, even desirable if you are not decanting. Since the OP is going to "2 step" this though it will certainly take longer than 24 hrs. and probably get decanted. For others making starters, pitching a starter at high krausen or a little after has it's adherants (at least one : ) I'd personally buy 2 vials for a starter and do a single step...unless the OP is brewing something exceedingly massive or just wants to practice stepping. up.
I've actually had a 'blow-off' before with a starter. It was with California Ale, built-up twice. Came to check on it the next morning and my foam stopper was on the ground, with a volcano of foam out the top of the flask. Getting a lot of bubbles is a lot better than getting no bubbles at all. You'll get a healthy ferment in the end.
I had quite a few, until I added a 5 Liter flask to the arsenal. Even that one has had blow-off once or twice with stepped starters.
