I reckon it's perfectly acceptable to hold out until 12 weeks - that is the 'traditional' time when it is considered safe to tell people. You're still giving them the better part of six months to find a replacement, if that isn't enough time that they can't run a business without you, then they need a management overhaul!

Don't let anyone make you feel guilty, this is your personal life and your decisions shouldn't be influenced by what someone else did.

Kerrie