So I'm 20 weeks and I have gained 10 lbs so far.  6 of those, I gained in a week!  But I haven't gained anymore this past week. I still feel so tiny, although the babies are growing fine so far.  I'm just curious to hear when others started "packing on the lbs".  
 
                TTC #3 (#1 for DH) since September 2011.  DX:  Unexplained infertility with possible cervical factor.
May 2012 did clomid with IUI - BFN
August 2012 IUI #2 w/ follistim/letrozole - BFN
September 2012 IUI #3 w/ follistim/letrozole - BFN
October 2012 IUI #4 w/ follistim/letrozole - BFN
November/December 2012 IVF/ICSI #1 Retrieved 12 eggs, 8 fertilized.  Transferred 1 beautiful embryo and 5 made it to freeze.  Got my BFP!  
January 2013 u/s showed empty gestational sac.
FET cycle April 2013.  Transferred 2 5 day embryos.  BFP!!!!!  2 heartbeats!!!!  EDD:  12/27/13
             
        
Re: when did you start gaining weight quickly?
By 20 weeks with my first I probably gained the same as you. I wasn't in maternity clothes until close to 6 months.
I would say that the weight gain was sudden instead of gradual for me, but now it is quick!
I agree! I'm 28 weeks and I've gained 12-14 pounds so far. I'm overweight so my recommendation is about the 35 pound mark too. I think I'm starting to gain more quickly now and from what I understand, many gain the most in the last trimester. We'll see... my main concern is that the babies are healthy!