Check out what your employers rules are on this matter. Mine asks for a minimum of 10 weeks notice in writing. If I'm lucky enough to get to 12 weeks (which seems unlikely right now) I will tell them then.

My company is notoriously slow at employing people so I want to ensure I give them plenty of time to get someone to replace me for the few months that I intend to take off. Ten weeks would not be enough for time for them to recruit & train a temporary person.... and I don't want to come back to chaos. Sounds ridiculous but they still haven't replaced me from my previous position that I left 5 months ago. Believe it or not this is a large company.