Our microlight guy installed a Trig mode S in his Ikarus in a couple of hours using a spare round hole in the panel.
If you're installing in an EASA aircraft you're probably going to need to get an avionics workshop to do it for you, including all the paperwork. One of the Trig transponders is plug and play for a popular older American item, but still needs setting up for the particular aircraft.
These are the 2 ends of the spectrum. Depends where your aircraft is in the range.
TOO