That’s an interesting question and there are some subtleties to answering it. The following are my opinions, not really clear Church teaching.
In general, NFP may be used to avoid pregnancy for as long as the just reason(s) to use it exits. This includes the option to use it indefinitely.
From Humanae Vitae:
With regard to physical, economic, psychological and social conditions, responsible parenthood is exercised by those who prudently and generously decide to have more children, and by those who, for serious reasons and with due respect to moral precepts, decide not to have additional children for either a certain or an indefinite period of time.
But, we could note here that it refers to “additional children”. Does this imply that it does not hold if the couple have not yet had any children? No, but it’s not entirely clear either way.
Obviously (I hope it’s obvious), a childless couple who find that there is a very serious reason not to have children are not bound to have children. eg if the mothers health will be gravely risked (including mental health).
But, there is a general responsibility for any married couple to be open to life; to procreate if it is possible. So we might say that in general, the reasons to use NFP indefinitely in the case of no children must be very serious indeed - the bar should be set higher than for couples who already have children. Procreation is rightly considered the primary (first, but not only) end of marriage, so a couple is generally bound to have children.
We can also note that a couple who already have children usually have more inherent reasons to avoid further children, such are the financial, physical, mental, emotional demands of parenting, that a childless couple do not yet face. So the use of NFP is generally more likely to be justified for those who are already parents.
Anyway, it’s a complex matter. I’m sure you’ll hear different opinions expressed here, either way. A trusted priest may be the best person to help you work through your own circumstances, taking into account any relevant medical advice you may have received.