Time management is one of the things you're assessed on, but there's not really a time limit. The only restriction is the exam board deadlines for when the project has to be submitted for moderation, which I think were November and May depending on which exam period you were submitting for (if that makes sense). So I started research and planning over the summer holidays between Year 12 and Year 13, then started writing the project around the start of term in September and had it finished a week or two after October half term. The way my school organised it, half the year was meant to submit in November and the other half in May, so we could choose when we wanted to write the project, although in practice most people in my group missed the November deadline and had to submit in May anyway. It shouldn't take you more than a couple of months to do, but it depends on what kind of research you need to do in preparation. Throughout the project you have to arrange meetings with your supervisor (probably a teacher at your school, either a form teacher or subject teacher in a relevant field) and fill out a kind of log of how the project is progressing and what changes you've made to your original plan - it's important to fill this in carefully, as a large proportion of your overal mark depends on it. Your supervisor will probably suggest deadlines for stages of the project, e.g. when they want a complete first draft, but it is extremely flexible.
I think in terms of UCAS tariff points an EPQ is equivalent to an AS level, which I'd say is about right given that it doesn't take very long to complete. Personally I didn't need the points as I was already doing four A-levels, but it makes for a good personal statement and is also a useful talking point if you have interviews.