What are the steps youve taken thus far? When harvesting from bottle dregs you need to start small, ~200ml of 1.020 wort, you probably wont see much activity with that step but it should get things woken up. Then feed it 500ml of some 1.040 wort and see if things progress, you'll see some activity and if not you should see a yeast layer on the bottom of the vessel. After that you should probably step it at least one more time with 1L of 1.040 wort, by which point you may have enough for a batch, but its all a guessing game.
You have to evaluate by how the yeast reacts when you feed it. You really cant build it up too much, but you can do it too fast so starting low and slow is the way to go.