time before departure

if my flight is at 11.45 a.m., when should I be at the airport?

The airport : which one ?

My flight is from Liverpool

The Liverpool Airport
http://www.liverpoolairport.com/passenger-information/before-you-arrive-at-the-airport.html
recommends that passengers arrive at the airport no later than 2 hours prior to departure.

But all airports recommend that, and it's often possible to arrive a bit later and still be on time. Depending on the affluence at that time and bearing in mind that the drop-desk closes 40 minutes before the scheduled departure time - and that you must be at the boarding gate at least 30 minutes before the departure.
If you are not an EU/EAA citizen, you have one more formality to comply with :
Irrespective of a passenger's visa requirements, all non EU/EEA citizens must have their travel documents checked and online boarding pass stamped at the Ryanair Visa/Document Check Desk before going through airport security.

I don't know Liverpool Airport, but it's not a very large airport. Were I to take your 11.45 flight, I would try to arrive in the terminal around 10.15 (with a checked baggage) or 10.30 (hand luggage only). Better safe than sorry !

thank you very much