You can buy it on the first day you intend to use it.

A mobile pass is usually issued within hours, a paper pass if sold at a ticket office is yours immediately but paper passes ordered from interrail.eu seem to take a very long time to be posted.

How far in advance is also dictated by the possibility you will have to provide a pass number for certain reservations, depending on where you go and where you gt reservations from.

IN NL kan je de paper versie ala minute kopen bij NS-INtern in Ams-CS en Schiphol-niet meer bij de overige-wel doen ze allemaal de mobiel. Wel alles met 7,50 extra kosten

Ook Treinreiswinkel kan het je verkopen-met toerekenen van posttermijn ook nog op verzoek op paper.

You can buy the pass (mobile) in the last minute but depending on where you want to travel you might need to do some reservations months, or weeks in advance. Like the Eurostar between UK and mainland Europe where passholder seats on popular departures can sell out months in advance.