One of my customers has asked me if there is a time limit (from PayPal) on paying one of my invoices. If there isn't, I can invoice her today and she will pay me in a couple of weeks. If there is a time limit, I'll need to schedule the invoice on my calendar for a later date. (Incidentally, I know this customer, and I trust her; so there's not an issue of scam . . . just of time).
Don't think that the time limit is officially stated anywhere. At least I have not seen it anywhere. As you can cancel an invoice at any time unless it has been paid, it's possible there is no time limit. Worst case scenario, should one expire, you would have to send another.
Another option, save the invoice as a Draft ... then send when ready.