K.G.
First I want to say way to go!! Both Breastfeeding and NFP are great for you, your child, and your marriage!!
If you are exclusivly BFing it is unlikly that your fertility will return before 6 months post partum...but it does happen. There are more reliable ways to chart your fertility that the "calender method". You can take you waking basil body temp and check for cervical mucus. When NFP is used properly it is 99% effective in preventing pregnancy.
Check out this web site for more info. nfpandmore.org
Good Luck and Many Blessings, K.