code: @Nullable
      public Long getExpireAfter(final TimeUnit timeUnit) {
          if (expireAfterSeconds == null) {
              return null;
          }
          return timeUnit.convert(expireAfterSeconds, TimeUnit.SECONDS);
      }
docs: Gets the time to live for documents in the collection
  
  @return the time to live for documents in the collection
  @param timeUnit the time unit
  @mongodb.driver.manual tutorial/expire-data TTL