Dont trust any interviewer saying you will get command in any time period (18months) unless its put in writing.
I must stress that any HR department will not want their FO's getting Foxtrot Oscar and leaving as this is a cost to the company and gives them more work. Airlines should be honest and open about their recruitment policy, and the ability to progress. For example when a new type is introduced its logical that there will be an intake of direct entry Captains and FO's but a good employer will also ringfence a given number or positions for internal candidates. However this gives rise to a question of what to do with FO's who will never make command?