You can take off your resists as soon as you have done your final dips in the indigo vat, and before you let the fibre oxidize for the final 24 hours.
If you're doing simple and random resists such as elastics, you can leave them on from the first dip right through to the last, or you can experiment by taking them off in between dips and putting them back on in different formations.  This way you get some areas that stay white, and some that have different amounts of dips, which results in varying shades of blue.