You'll be okay until you reach 12 o'clock. Where are you coming from in the East? I get on at Junction 29 for Basildon/Southend and it's usually not too bad in that area.
I live in Chelmsford so join at the A12 and often go to Heathrow. I would leave 2 hours for my journey. Its got better as the roadworks have reduced but you always have to plan for the car park situation!