How long should chia seeds soak?
I usually let it soak overnight, though it isn't necessary to soak that long. You should soak it till it forms a paste/gel like consistency and it would take at least  2-3 hours, though this website claims under ten minutes (time might vary depending on the batch of the seeds). You could also soak it in any juice or fruit extract as well.

Additionally, you can also use it in its raw form without soaking, the way one would use poppy/sesame seeds.