Jet2's check-in desk at Stansted opens 3 hours before your flight's departure time and closes 40 minutes before it:
https://www.jet2.com/FAQs/airport-information/london-stansted
You only need to go to the check-in desk though if you've got hold baggage and/or you've not checked in online. If you've only got hand luggage, and you've checked in online, you can go straight to the security gate queues.
Stansted's advices is that "as a general guide, you will need to have checked in and be ready to go through security at least 2 hours before your flight departure time"
https://www.stanstedairport.com/help/passenger-guides/checking-in/
Stansted offers fast track lanes for security/passport clearance, both inbound and outbound. However, whenever I've been at Stansted, the regular security queues haven't been any longer than the fast track ones anyway:
https://www.stanstedairport.com/at-the-airport/security-fasttrack/