It varies for every person. I was exclusively breastfeeding my 2nd son and since my husband and I were done with our family, even though I had not gotten my cycle back yet, we were using condoms 100% religiously. Needless to say it was a HUGE surprise when I became pregnant with our third baby (a girl!) when my 2nd son was 5 months old. It was such a surprise that I had no clue I was pregnant again for over 3 months.
Check out this website: http://www.freeopks.com/ you can get 10 FREE ovulation tests or 10 free pregnancy tests or any combination that equals 10- you pay nothing at all and do not give any credit card information or anything. It is from Sacred Heart Mission- a non-profit interested in helping people become pregnant? Anyway, I just ordered the ovulation tests so I can NOT get pregnant- 3 is more than enough for us:) I just ordered them and haven't got them yet but am excited!
I am a 38 yo stay at home mom with a 6 year old son, a 17 month old son and a 2 1/1 month old daughter. My wonderful husband and I have been married 12 years:)
Good luck in expanding your family!