It varies a uge amount from woman to woman. Some people get their periods back 4 weeks after birth despite exclusive breastfeeding. For others at the other extreme they have to stop completely (even if their baby is a toddler) before their fertility returns. For most women it's somewhere in between. In fact, one of the commonest times for periods to return is after solids are introduced at 6 months.
The bigger the gaps between feeds and the more of things other than breastmilk that a baby consumes, the more likely the mothers fertility is to return. Exclusive breastfeeding round the clock with no other solids or liquids, no big gaps between feeds and no periods is up to 98% effective as a contraceptive, i.e. only 2 out of 100 women will get pregnant under those circumstances. Change any one of those factors and the contraceptive effect reduces (which is why it is often not recommended as contraception and lots of people know someone who fell pregnant while bf -these conditions are rarely all met by UK mums).
Making a big sudden change to breastfeeding pattern is recognised as something more likely to trigger the return of fertility than gradually cutting back. Hence why solids is such a common trigger. for me with ds1 I first ovulated a few days after stopping expressing at work just before he had his first birthday. With dd it was when she stopped feeding so much at night fairly suddenly at around 14 months.
There's more info on Kellymom here that you might find useful.
I'm sure others will talk to you about their personal experience of weighing up the needs of one existing child vs another potential child, especially when age comes into play. It's such a personal decision. Small age gaps are sometimes hard work and there's also the consideration of putting your body through another pregnancy when you are also caring for a young baby. This is extremely physically demanding, especially as in the first year of your baby's life your body is still recovering from the last pregnancy.
You might also want to take into account that the biggest health benefits from breastfeeding come from the period when milk is the most important form of nutrition before solids are introduced, i.e. first 6 months, and that the good effects of breastmilk are generally dose-dependent, i.e. the more breast milk the better.
If you conceive, then feeding during pregnancy is possible (I'm currently doing it for the second time), but with a baby under 1 year for whom milk should be their main source of nutrition, and even mroeso under 6 months supplementation may be required, as in the majority of women the pregnancy hormones reduce the amount of milk produced, sometimes v significantly.
Good luck with your decision .