The time scale is from several months to over a year. It takes a while to answer questions like - does it provide immunity, is it safe, what doses provide immunity and are safe, is one innoculation enough or does it need to be repeated, how long does the immunity last and so on. When you come across headlines about institutions which have began vaccine trials (there have been a few), that means that they are in the early stages of answering these questions. Then you have questions of production and distribution which can vary a lot depending on the type of vaccine that's produced.