The timespans for medieval building projects are not so much dependent on size but on funding, which would be mainly from private donations. If the local lord wants to pay for the whole church, then it could go up in a few years. If there's trouble raising money it could get stretched out for much longer. Presuming we're talking about a stone building, usually they'd start at one end and if the project was taking a while build a temporary structure and/or roof made out of wood to accommodate the worshippers.