You can check in for flights (including your return flight) up to 28 days before departure.
Quote:
"If you’ve chosen to check in online, you can do this from 28 days and up to 5 hours before your departure. You can check in via the website or the mobile app. Please remember to download your mobile boarding passes on the app onto your smartphone or alternatively print your passes on the desktop site. If you’ve got hold baggage to check in, simply go to the Jet2.com check-in area (look out for our friendly staff in red uniforms) when you arrive at the airport. Remember that our check-in desks close 40 minutes before your departure. If you’re travelling with just hand baggage, you can go straight to airport security with your boarding pass."
Source:
https://www.jet2.com/faqs/check-in-mobile/#gsc.tab=0