Early Morning Flight From Heathrow - how early to arrive?

How early should I arrive for a 7:15am flight from Heathrow to Oslo on a Friday morning?  I've only ever flown in and out of Birmingham where I can arrive less than an hour before my flight and have plenty of time to check-in but I know Heathrow is very different.  I'm just not sure how different.  Thanks in advance for your advice!

can you use online checkin? 

can you use online checkin? 

Yes

An hour and 20 minutes max.   The airport is surprisingly quiet at that time and if you have your boarding pass printed before you get there and are just dropping off bags to one of the MAJOR airlines.  You shouldn't have a problem.  I used to travel for work quite a bit on those early morning flights and was able to generally get through with no problem with as little as 45 minutes. 

Thank you, that's very helpful!

Totally agree with karin.  I have become really really bad though and have been leaving only 30 minutes...which means I actually missed check in for a flight to Belfast last month.  It was okay in the end but I had to wait 4 hours at crappy Stansted.  I have LEARNED MY LESSON now!  Of course, I've only been allowing about 30 minutes at Edinburgh on my beloved Air France flights...I am getting slack about it again!