I am currently trying too encode my AE project to .mp4. It is a 60 second short film and it has been encoding for 14 hours and 7 hours remaining. I've tried to change the renderer but the option is unavailable. I"ve tried absolutely everything possible but nothing is working.
My first guess is that you are new to this and my second guess is that you have Ray-traced rendering set in one of your compositions.
Without workflow details, composition settings, and a lot more information the 2 or 3 billion different combinations of things you could possibly do in After Effects make it pretty difficult for us to figure out what's happening.