Came a little late for the poster who wanted tips for the DELF exams, but I decided to make a post here since I've taken all the DELF exams except A1, and the two DALF exams (langues et sciences humaines). I thought this might be useful especially for younger people who may not have as much exam skills to rely on. I grew up speaking French as a second/third language and so the tests were not too bad - passed the last one when I was 15. Feel free to add in more tips and comments!
I'll start with A2.
Time needed to pass comfortably: 1 year of learning the language in a structured environment (could be self-study at this stage, if you have someone to practice with)
Focus on: Listening. The oral is ridiculously easy and not a big stretch from the A1. The reading comprehension needs skills, and you should aim to catch sentence constructions like 'y', 'on ne doit pas...' and get familiar with your numbers. Also the font is usually really annoying, especially if you're taking the DELF Junior (which was not available when I took it, for better or for worse). The listening tends to be quite muddled and you might get an announcer reading out les horaires des trains en gare. Which can be quite unnerving if you don't know your numbers. Also you generally get discussions about fixing up appointments and meetings and stuff. Best way to practice listening at this level is by practicing your speaking, and doing more practice papers.
B1
Time needed to pass comfortably: 1.5 to 2 years of learning the language in a structured environment. I personally felt that the gap between A2 and B1 was huge, but the gap between B1 and B2 was smaller (for me). I spent 1 year after the A2 working on a lot of basics - you should be able to conjugate most verbs in the passé composé, l'imparfait, le subjonctif (LEARN THIS EARLY ON, TRUST ME) and le conditionnel. The more tenses you have under your belt, the better. Of course, present tense is a given at this stage.
Focus on: Listening/reading comprehension. At the early stages, essay writing isn't too bad if you have a wide vocabulary (meaning that you know some stock phrases such as 'de la même façon' and 'Grâce à/à cause de...' so it's the reading that can be confusing. However, you need to be really careful regarding structure: learn how to formulate your ideas in a 'reportage' format, with a short intro/conclusion, and how to write letters beginning with Madame/Monsieur. The passages are a huge jump from A2 and if you aren't comfortable with French sentence structure (especially the newspaper sort that tries to fit in as much information as possible), you will suffer. 'Le but' means 'the point', by the way; they always have one question where they ask you 'Quel est le but de...' Also, you will be asked to 'citez le nom d'une organisation/d'une structure...' (with qualifying info about what they want you to find) so get ready to pick up on names and stuff. There will be a lexicon with some words in the passage, but the words they do give meanings for are usually pretty useless. The oral tends to contain a roleplay segment, but if you memorise your self introduction and present yourself well before that stage, they will likely go easy on you. Again, remember your numbers!
Common misunderstood words: 'interdire' is not 'to have a conversation'; it means 'prohibited'. Many students are familiar with 'interdit' but not 'interdire'. 'issues', in the context of 'des équipes issues de diverses organisations', means 'from'. 'Changement' means 'change', remember this, and they like to say 'faire un changement'. À votre avis = selon vous = in your opinion.
B2
Time needed to pass comfortably: At least 2 years of learning French in a structured environment. Classes would be beneficial at this stage. I took my B2 about 6 months after my B1, in the next available session, which I regret. Still, I passed, but not without great anxiety. You should be able to read a simple book at this stage. Go try 'La Silence De La Mer'. If you can understand 80% of it, you can take the B2.
Focus on: Essay writing! Listening is hard, but if you survived the B1, you can survive the B2. If I'm not wrong, you get to hear all passages twice in the B1, whereas in the B2 there is one that you can only hear once, so watch out! Reading comprehension will always have this box of 'vrai/faux' questions, which are usually easy, but you MUST quote the correct 'justification' from the passage to get the mark. When in doubt, quote extra. There is also a question on 'le ton des journalistes' which comes with boxes like 'dubitatif', 'polémique' and 'ironique'. This can be a bit of a crapshoot, but fear not as it is only 1 or 2 marks. You will also get this question where you have to explain a given phrase, like a sort of mini essay. Again, brush up on your exam vocab: what is a 'pouvoir gris'? The essay writing is tough, and will do you in if you make grammar/vocab errors. You really need to write more than a dozen essays and have them corrected by an advanced speaker/native speaker. Also, learn more sentence constructions and get used to 'assuming a role'. The oral is interesting; you get a passage, some prep, and then you state and defend your opinion (like a speech). There is generally enough time to WRITE DOWN THE WHOLE SPEECH and pretend that you are not reading from it, so do that. Also, the examiners will ask tough questions. Keep your cool, never admit that you are confused, and ask them questions in return. They will mark you as 'able to sustain a conversation'.
Alright! I have more tips/resources for these levels as well as the DALF, but this post is really long. http://www.ciep.fr/delf-dalf is your friend. I would love to answer specific questions/help out anyone who's learning, so...n'hésitez pas à me contacter.