I think timing a verse and multiplying by the desired number of verses, for each carol, then adding 30 seconds between each and then a couple minutes' overall buffer, is enough. It has worked well enough for me. If you haven't done it before you might find you can't fit as much into 30 minutes as you might naively expect.
Really depends on how many verses you sing.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.