3 years is a good timescale but it depends on what time off you can get at work. Also it may be worth asking about unpaid leave, even if its just one month.
I spent my hols either in the US flying (and you will be amazed at how many hours can be fitted into how little time) or on a pre-exam brush up course (well worth it if you've been self studying at home).
I agree with what people have said about planning but the main thing I would add is to say that you must be able to prioritise. When you are a month away from an exam or flight test or doing an intensive part of your training such as the IR you need to have NO distractions. NONE! You will have to ignore anything and everything that isn't directly related to the flying.
It can be difficult yes but its the main thing you need to do. I have seen the result of someone being distracted by things that don't matter.
So sort out things like paying of household bills, all that sort of thing, in advance. Clear the decks for the cramming.
By the way, it took me around 3-4 years to complete my CPL/IR whilst working full time, it took total dedication in both time and money, which can be very very hard, but I got a jet job straight after getting my licence, and when you are on your first day at work in a shiny jet it is sooooo worth it!
If you get fed up, come to Pprune for some motivation!