Nobody can predict what your body will do. You may return to normal cycles quite quickly, or breastfeeding may continue to affect your cycles for quite some time or even until after you wean. In my own experience, I had some pretty long cycles with few truly safe days, and we had what we felt was a very serious reason for avoiding pregnancy (pregnancy would almost certainly result in miscarriage due to luteal phase defect), so we were pretty conservative, but I was toying with rules that would have given us a tight fertility window and probably prevented conception despite having long and irregular cycles. I don't know if it would have held up over time, though, as we switched to TTC for my 6th postpartum ovulation and got pregnant that cycle (even though I didn't ovulate until 5 weeks into my cycle!).
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The rules I was toying with were that the fertile phase would begin on the first day of "more fertile-type" mucus (defined as clear, stretchy, or slippery) until I had a sympto-thermal rule. On the cycle with my second postpartum ovulation, this wouldn't have helped me a whole lot. I had more-fertile mucus beginning on CD 13 and had 2 weeks more before I had a thermal shift, but my peak day wasn't until 4 days after that, and my period started on P+4, so I had no post-ovulation infertile days that cycle. However, the next 4 cycles, I had 6-9 days from the start of more-fertile CM (not all days in between were more-fertile type CM) through peak day/before temp rise. You really don't want it to be under 6 days if you've got less-fertile type mucus, or you really increase your likelihood of getting pregnant. You'd want to be careful about the "not in the morning" (so you can make adequate observations) and "not on consecutive days" (so that you can make observations without seminal residue) rules for the pre-ovulation infertile phase.
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I wouldn't guess that every woman would have the same experience as I did, and I wouldn't expect my "rules" to yield anything like the effectiveness that some people need. You might consider following the rules and observing what your body does around ovulation for a few cycles and see if you can see any patterns to make rules about what you're comfortable with.