Timings for visa Application

My husband and I are going on a World Cruise from 10th January 2017, we have been told we will need a double entry permit. we arrive in Shanghi on 22/3/17 for day and again in Singapore on 8th April for 2 days overnight on ship. When is the earliest we could apply for it as understand they are only valid for 90 days and we need it before we leave the U.K. on 10th January.