A system of Large Observing Proposals was introduced at the recent Call for Proposals for Period 51 (for the observing period April 1, 2015 - October 1, 2015) with deadline November 17, 2014, where researchers or research groups can apply for observing time for multi-semester programs. The general rules and requirements applicable to Large Observing Proposals are given here:
http://www.not.iac.es/observing/largeproposals/